IR-U DOUBLE RESONANCE SPECTRUM OF ACETYLENE BELOW AND ABOVE THE PREDISSOCIATION THRESHOLD

"1 The 1A <--X Xg electronic transition of the vibrationally excited acetylene molecule was studied by IR-UV double resonance spectroscopy in gas and in a supersonic jet. The C-H antisymmetric stretching vibration VCH in the A state was clearly observed when the molecule was excited to the VCH + VCH combination vibration in the X state by the IR laser. When the VCH fundamental vibration was excited, the C-H in-plane cis-bending vibration Vcis(in) in the A state was observed strongly, while VCH almost disappea_red. The difference was interpreted in terms of Fermi resonance of the VCH fundamental vibration in X. The predissociation threshold was newly determined to be 46,439-46,673 cm (133.11 + 0.33 kcal/mol). In the region above the predissociation threshold, strong vibrational mixing was found. The higher members of the progression of the trans-bending vibration starting from VCH were assigned. It was suggested that the nonradiative relaxation accelerated in the region above 51,744 cm-1.


INTRODUCTION
The supersonic jet technique enables us to study the vibrational and rotational struc- tures of an excited electronic state of a molecule in detail.Most molecules in a supersonic jet are populated in the zero point level of the potential surface in the ground state, thus the spectral congestions due to hot bands are eliminated very well.As a result, the spectrum shows a simple, well-resolved rovibronic structure, which is analyzed easily.In the last decade, this technique has revealed the structures of chemically important large molecules in the excited states.However, there are still few molecules of which all the vibrational modes have been assigned in the excited 162 MASAAKI FUJII et al. electronic state.All the vibrational levels in the excited state do not always appear in the electronic spectrum because of the Franck-Condon factor from the zero point vibrational level.For example, usually the rrrr* electronic transition does not show a vibronic band of the C-H vibration in the excited state.C-H bond length does not change much in going from the ground state to the rrrr* excited state, therefore the C-H vibrational level does not have sufficient Franck-Condon factor in the rrrr* transition.
This restriction can be removed if we can populate the molecules to a specific vibrational level other than the zero point level.The transition from the excited vibrational level will have the completely different Franck-Condon factors for the vibronic levels in the excited electronic state.Consequently, hidden vibronic levels in the excited state will be observed in the electronic spectrum of the vibrationally excited molecule.For example, the C-H vibration in the rrrr* state will be clearly observed when the molecules are populated in the C-H vibration in the ground state.The electronic transition of the vibrationally excited molecule is important not only for the spectroscopy but also for the dynamics.The molecule in the overtone vibra- tional level will show an intense transition to a higher vibronic level in the excited state, where the relaxation and the photochemical reaction will be activated.
Recently, two possibilities of the vibrational excitation have been proposed.One is stimulated Raman pumping and another is, of course, excitation by an IR laser.The stimulated Raman pumping can be achieved by two lasers in the visible region, such as the second harmonics of a YAG laser and the YAG pumped dye laser.A visible dye laser is a popular light source, therefore the stimulated Raman pumping and the spectroscopy of the resultant vibrationally excited molecule have been developed well.This spectroscopy, called the stimulated Raman-UV optical double resonance, has enough high sensitivity and pumping efficiency to manifest the transition of a vibrationally excited vdW complex in a supersonic jet. - Furthermore, the symmetry of the vibrational level can be determined by the depolarization ratio of the stimulated Raman process.On the other hand, IR excitation has not been well developed.At the present time, tunable pulsed IR laser light has been obtained by the frequency conversion of the dye laser light by the H Raman shifter or the differential wave generation by a nonlinear crystal.Since the conversion efficiency is only a small percentage of the source, the experiment becomes difficult.Consequently, the spectroscopy of the vibrationally excited molecule by the IR laser (the IR-UV double resonance spectroscopy) has been reported only for benzene 8,9 and acetylene, 1 where the latter was used to study the collisional relaxation in the ground state.However, this does not mean the advantage of the stimulated Raman pumping.Since the Raman pumping and the IR excitation are complementary for vibrational excitation, both pumping methods are necessary.This is specially so because the C-H vibration has a large absorption coefficient for IR, while it does not have a large cross section for the Raman process.Therefore, the C-H vibration in the excited state is the best target for IR-UV double resonance spectroscopy.Another target is the spectroscopy and dynamics of the higher vibrational levels in the excited state.The pumping efficiency of the nonlinear Raman process decreases for overtones in comparison with that of the IR excitation.Therefore the IR-UV double resonance will enable us to observe the higher vibrational levels in the excited state rather easily.[3][4][5][6][7][8][9] In the present work, we applied the IR-UV double resonance spectroscopy to the / state of acetylene.The molecule was excited to the C-H antisymmetric stretching vibration VcH "nt or the combination vibration of VCH ant and the C-H symmetric stretching vibration vcHSy by the IR laser of-3 pm or-1.5 pm, respectively.One of the purposes is the observation of the unassigned C-H vibrations, vc.a", the in-plane and the out-of-plane cis-bending vibrations Vcis(in) and Vci,,(out) in the A state.For this purpose, we excited the molecule to a specific rovibrational level in the ground state by the IR 1.aser, and probed the lower vibrational region in the/ state.The acetylene molecule has high fluorescence quantum yield in the lower ener- gy region of/, therefore the transition was detected by the fluorescence (the IR-UV double resonance LIF spectroscopy, see Figure 1).Another purpose is related to the predissociation of acetylene in the A state.We have found the predissociation threshold in * between 46,339 and 46,673 cm -1 from the drastic decrease in the fluorescence quantum yield. 2 The rotational level dependence of the fluorescence quantum yield shows that its mechanism is the vibrational predissociation to C2H + H. Based on this background, we observed the IR-UV double resonance spectrum in the region above the predissociation threshold to obtain further information on the predissociation mechanism.In the region above the predissociation threshold, the LIF detection is no longer available because of the decrease in fluorescence quantum yield.Therefore the double resonance transition was detected by the multiphoton ionization signal (see Figure 1).The molecule was excited to the VCH sym + VCH ant combination vibration in X.The vibrational motion of VCH sym if" VCH ant is close to the dissociation coordinate C2H + H, therefore we expected the transition to a vibronic level related to the predissociation.
Very recently, Crim and co-workers have reported the near IR-UV double resonance LIF spectrum of acetylene via 3VcH ant, and have assigned vcis(in), vcis (out) and VCH ant in the / state. 21,22Our assignments are consistent with theirs, and two independent observations via different vibrational levels confirm the assignments of C-H vibrations in the/ state.

EXPERIMENTAL
The experimental apparatus for IR-UV double resonance spectroscopy is the same as that for UV-IR double resonance spectroscopy. 23Two dye lasers (Lambda Physik FL3002 and Quantel TDL 50, respectively) were pumped separately by a XeC1 ex- cimer laser (Lambda Physik LPX 100) and a Nd3+:YAG laser (Quantel YG 581-10).The excimer laser and the YAG laser were triggered by the same pulse generator and were adjusted to give a 30 ns delay for the excimer laser by a digital delayedpulse generator (BNC).The UV laser light Vuv was obtained by frequency-doubling of the excimer laser pumped dye laser (dye: C500, C480 and C460) in BBO crystal, and was used to probe the N-X electronic transition of acetylene.The tunable IR laser light v was generated by differential mixing with the output of the YAG pumped dye laser (--640 nm, DCM) in LIO crystal (Inrad).Two kinds of mixing methods were used to generate v depending on its wavelength region.For the gen- eration of v in the 1.5 m region, the dye laser light was differentially mixed with the fundamental of the YAG laser (1.064 m).The differential wave v (<0.6 mJ) was used to excite the acetylene molecule to the overtone of CH vibration in the X state (VCH sym + VcHant).For the 3 tm region, the dye laser light was mixed with the second harmonics of the YAG laser (532 nm).The differential wave (--30 J) was used for the excitation of the fundamental vibration of CH antisymmetric stretch- ing.Both v and Vuv were coaxially introduced into a gas cell or a vacuum chamber which has a pulsed nozzle source for a supersonic jet.The acetylene molecule was excited to a specific rovibrational level in by v and was further excited to the / state from the vibrationally excited level by Vuv.The wavelength of v was fixed by measuring a photoacoustic spectrum or an infrared absorption spectrum of acetylene gas in moderate pressure (--200 Torr) by v.The detail of the photoacous- tic spectrum of acetylene by the pulsed tunable IR laser has been described else- where.In the region below the predissociation limit (42,000-47,200 cm-1) in the / state, the /-i electronic transition of the vibrationally excited acetylene was detected by fluorescence from the N state (IR-UV double resonance LIF spectrum).
The IR-UV double resonance LIF spectrum was measured in low pressure gas (10--100 rn Torr) at room temperature.The total fluorescence was detected by a photomultiplier (Hamamatsu R-928) through an IR cut filter (Toshiba IRA-25S) at 90 degree from the lasers.In the region above the predissociation limit (49,500--52,500cm-1), the transition was detected by one-photon resonant two- photon ionization signal (IR-UV double resonance MPI spectrum).IR-UV double resonance MPI spectrum was measured in a supersonic jet of pure acetylene gas (1.5 atm).Ions generated by vr + 2Vuv were pushed by a repeller at an appropriate voltage (typically 10 V/cm) into a detector chamber, and were detected by a channel multiplier (Murata Ceratron).Both the fluorescence and the ion signals were amplified by an amplifier (NF for the fluorescence and Keithley for the ion) and were integrated by a digital boxcar system (EG&G PAR 4402/4420).The integrated signal was recorded by a microcomputer (NEC).
In the present experiment, the double resonance signal was observed with the one-color signal due to Vuv only.To distinguish the double resonance signal from the one-color signal, we employed an alternative data acquisition system which was fully described elsewhere. 23Briefly, the excimer laser was operated in 20 Hz while the YAG laser was triggered in 10 Hz synchronously.Consequently, the signal due to viR + Vuv and that due to Vuv only appeared alternatively.Each signal was separately integrated and was stored in a different memory array.Thus the spectrum due to v + Vtjv and the spectrum due to Vuv were obtained at one time, and the double resonance signal was easily distinguished from the one-color signal by com- paring both spectra.Acetylene was purchased from Teisan, and was used after passing through a solid CO2 methanol trap.

Notation of Vibrational Mode and Selection Rule
It is convenient to summarize notation, symmetry and selection role for the transition of the acetylene molecule.5][26][27] Because of this geometrical change, a long and intense progression of the trans-bent vibration appears in the/3i-X transition.If the traditional notation vl, v2 is used, the notation of vibrational mode changes in i and in ) because of the geometrical change.For example, the C-H trans bending vibration is v4" in X but is v3' in/3i.To avoid confusion, we use the following notations: Vcn ym for C-H symmetric stretching vibrational mode, Vcc for C-C stretching, V for C-H trans-bending, Vcn "nt for C-H antisymmetric stretching for both states.C-H cis-bending vibrational mode is denoted to be Vci in the state.In the state, this mode splits into in-plane cis-bending Vcis(in and out-of-plane cis-bending Vcis(OUt).The correspondence to the traditional notation is summarized in Table 1.
The symmetry of the vibrational modes is described by Dooh and C2h point groups in the X and A states, respectively.The symmetry of each vibrational mode is also shown in Table 1.When we discuss the symmetry restriction in the/transition, the symmetry species of Dooh must be resolved to species in C2h.For electronic sym- metry, Zg for the X state is resolved to Ag in C2h.Thus, the lA R1Eg band is allowed transition.The symmetries for the vibrational modes in R are also resolved to Czh and are also shown in Table 1.According to this correlation, the VcH ant fun- damental and the vcr?y + Vcr ant combination vibrational levels have ru symmetry which is resolved to bu in Czh.Therefore, the bu vibrational levels in/ are allowed in the transition from these vibrationally excited levels.
The geometrical change also affects the definition of the angular momentum.The angular momentum about the molecular axis K is defined to be K + g + A in , where g is the vibrational angular momentum g ag, a 2, a9 4 1 or 0, and A is the electronic angular momentum about the molecular axis.For example, both the VCH ant and VCH sym q" "VCH ant vibrational levels have K 0. On the other hand, K in the/ state is the rotational angular momentum about the molecular axis.Since the trans-bent structure in is a nearly symmetric top, thetransition is mostly explained by the selection rule of the perpendicular transition AK +1.Thus the K 1 vibronic sublevel will be observed strongly in the transition from the VCH ant and VCH sym "t-'CH ant vibrational levels in .
IR-UV Double Resonance Spectrum below Predissociation Threshold Prior to the IR-UV double resonance experiment, the frequency of the tunable IR laser vr must be fixed to a specific rovibrational level in the X state.For this purpose, the photoacoustic spectrum or the IR absorption spectrum of acetylene gas (100--200 Torr) at room temperature was measured for the fundamental vibrational band of VCH ant and the combination vibrational band VCH sym "" VCH ant by VlR.The photoacoustic spectrum of acetylene by the pulsed tunable IR laser was already reported elsewhere] and the rovibrational structures have been well analyzed by high resolution IR spectroscopy. 28,29Briefly, the VCH ant fundamental vibration shows com- plicated rotational structure due to Fermi resonance with (Vcc + V + Vcis) level, 28 where superscript 0 shows the vibrational angular momentum g.The intensity of the Fermi pair (Vcc + V + Vcis) is almost comparable to that of VCHant; therefore, the VCH ant vibrational level contains a substantial amount of the Vcc + V + Vcis component.In contrast, the combination vibration VCH sym -1-VCH ant shows a regular ,Y_,-Z type rotational structure. 7,28It means that this combination vibrational level does not have a strong Fermi resonance, and is mostly pure vibrational level.Therefore, the IR-UV double resonance spectrum via the fundamental VCH ant level is expected to show not only the VCH ant vibration but also Vcis vibration in the i state.On the other hand, only the band containing the VCH ant vibration will be found in the double 168 MASAAKI FUJII et al.
resonance spectrum when the combination vibration will be excited by VR.The former is convenient to probe unknown vibrational levels in /, and the latter is useful to confirm the assignment of Vcn ""t.By this expectation, we observed the IR-UV double resonance spectrum via the two vibrational levels.
Figure 2a shows the IR-UV double resonance LIF spectrum of acetylene gas at room temperature obtained after exciting the molecule to J"= 8 rovibrational level of the combination vibration 'CH sym " VCH ant in the state.The frequency of V'IR was fixed to P(9) line of the combination vibrational transition.The pressure of acetylene gas was kept at 10 m Torr to avoid collisional relaxation.Figure 2b shows the simul- taneously observed spectrum due to Vuv only (see Experimental section), which corresponds to the usual +electronic spectrum of acetylene in the hot band region (37,900-40,700 cm-1).The horizontal scale was drawn by the total energy, i.e. sum of the level energy of J" 8 of vcn sym + VCH "nt and the energy of the UV laser.From the comparison between them, four band groups due to V'IR "" V'UV are found.A weak band group at-46,500 cm -1 is shown in an expanded scale (Figure 2c).The three band groups indicated by solid lines consist of three lines, of which the spacing among them is 14 and 23 cm-.These values match the P(8)-Q(8) and Q(8)-R(8) spacing in the typical rovibronic structure in /-system (14.drawn in an expanded scale.The spectra were obtained by exciting the molecule to J' 8 level in the "VCH sym d-VCH vibration by VR.Assignments are shown by solid lines.The horizontal scales were drawn by the total energy measured from the zero point level of X. 23.13 cm -1 in the transition of V 0, K 1 <---X V 0, K 0). 27Therefore the three band groups are concluded to be the transition from the combination vibra- tional level excited by vR.The rest group at-45,600 cm -1 consists of four lines, and the spacing does not match the typical value shown above.Thus it is hard to conclude that tlfis band group is the/-X transition from the J" 8 of vibrationally excited acetylene.This group may be due to the transition to an unknown electronic state such as cis-bent state; however, we cannot make any definite conclusion at the present time.The observed frequencies for Vtv, total energies from zero rovibrational level in X and the excess energies from the origin of the/ state (42,197.57cm-1) are listed in Table 2. b Rovibrational energy in measured from the origin of , (42,197.57cm-).
c) Rovibrational energy of K 1, J level in , obtained from observed P and R lines.
d) Estimated vibrational energy by assuming the typical value (13.1 cm-) for the rotational constant A.
Let us discuss the vibrational assignments of three band groups due to the transition of the vibrationally excited acetylene.In our experiment, a small amount of molecules can be excited to the combination vibrational level because of the small absorption cross section.Therefore all the observed bands should be assigned to an allowed transition.As described in the last section, the level of bu symmetry is allowed in the transition from the "VCH sym q-VCH ant level (o-o in Dooh, bu in C2h ).Therefore the lowest frequency band group at 45,143 cm -(for Q(8) should be the transition to the fundamental vibration in bu symmetry.According to the symmetry correlation in Table 1, the vibrational mode in bu symmetry is the in-plane cis-bending mode Vcis(in) or the C-H antisymmetric stretching mode VCH ant.The bending vibration Vcis(in) does not match the substantial rovibrational energy (2,945.5 cm-) of this fundamental band.Therefore we conclude that this band is the transition to the fundamental vibration of VCH ant in /.The other two groups appear at 1,035 and 1,382cm -1 higher energy from the VCH ant band.The differences of 1,035 and 1,382 cm -1 are close to the vibrational frequencies of the trans-bending mode V (1,047.55cm-) and the C--C stretching mode Vcc (1,386.90cm-),respectively. 27erefore, it is concluded that the bands at 1,035 cm -1 and at 1,382 cm -1 are assigned to the VCH ant + V and V'CH ant + VCC vibration in the state, respectively.The strong appearance of the VCH ant + V vibration is consistent with the geometrical change in going from the linear X state to the trans-bent state.
To obtain the pure vibrational frequency of V'CH ant in the/ state, rotational energy must be subtracted from the observed energy of the "VCH ant vibration.It is best to measure rotational energy by measuring the IR-UV double resonance spectrum from the various J" levels in the combination vibration in X.Unfortunately, we could not pump the molecule to the rovibrational levels other than J" 8 because of the in- sufficient laser power of v.As a result, J' 1, K 1 level energy was obtained to be 2,871.1 cm from the observed level energies of P(8) and R(8).Using the typical value of the rotational constant A (13.1 cm for the zero vibrational level), the vibrational frequency of the V'CH ant is estimated to be 2,857 cm-.Similarly, the pure vibrational energy was also estimated for the VCH ant + W and VCH ant + VCC.The values are also listed in Table 2. Very recently, Tobiason et al. 2 have also observed the VCH ant vibration in /3i by the near IR-UV double resonance spectroscopy via 3VCH ant in X.From thorough rotational analysis, they have reported the frequency of VCH ant to be 2,857.4+ 0.2 cm-.Our value is consistent with theirs; therefore, the assignment of mode VCH ant in i is confirmed by observation from two different vibrational levels in X.
The region below the predissociation threshold is also probed from the VCH ant vibrational level in X. Figure 3a shows the IR-UV double resonance LIF spectrum of acetylene at room temperature in the total energy region from 43,300-45,300 cm-1.
The spectrum was obtained after exciting the molecule to the J" 8 level of the "VCH ant fundamental vibration in the X state by v.The collisional relaxation was negligible because of the low pressure condition (100 rn Torr).Figure 3b is the simultaneously observed spectrum without v, which corresponds to the hot band region of an ordinary LIF spectrum of gaseous acetylene.Clearly, two strong band groups are found at--44,000 and at-45,000 cm-, i.e. 1,800 and 2,800 cm from the origin of the /3i state.Both band groups consist of more than three rotational lines.It is a largely complicated structure in comparison with the spectrum via the combination vibration (Figure 2). Figure 3c and 3d show the same spectra shown in Figure 3a and 3b, respectively, but in an expanded scale around the band group at-45,000 cm -1 where the VCH ant vibration has been assigned.The positions of P(8), Q(8) and R(8) of the V'CH ant vibration are indicated by broken lines.As can be seen in the figure, the VCH ant vibration is also observed in this spectrum; however, its intensity is very weak.No error is expected because both the double resonance spectrum via the combination vibration and the present spectrum were obtained by exciting the molecule to the same rotational level J" 8. Instead of the weak VCH ant vibration, a new vibronic band with P(8), Q(8) and R(8) rotational structure appears strongly.The rotational assignments are indicated in the figure.In addition, the spectrum shows many weak bands, which will be discussed later.C) The IR-UV double resonance spectrum and d) the one-color spectrum around 45,000 cm in an expanded scale.Assignments are shown by solid lines.The horizontal scales were drawn by the total energy measured from the zero point level of X.
The strong intensity of the new vibronic band suggests that this band is also an allowed transition to the vibrational level in bu symmetry.Since one of the two vibrations in bu symmetry, vcHant, has already been assigned at a higher energy region than the position of the new band, this band should be assigned to a combination vibration inducing another bu vibration, Vcis(in).The strong intensity also suggests that this band contains trans-bending vibration V which gives larger Franck-Condon factor because of the geometrical change between X and/.Another intense band group at-44,000 cm -1 is about 1,000 cm -1 below the new band; therefore, it is highly possible to assign them to be the progression of mode V with the false origin of Vcis(in).
To establish the vibrational assignments, the IR-UV double resonance spectrum was measured in a wider energy region.Figure 4 shows a) the LIF spectrum due to VR + VUv, and b) the LIF spectrum due to Vtv only.Both  Total Energy / cm-1 Figure 4 a) A-X LIF spectrum of acetylene (100 m Torr) observed with vR + Vuv, b) simultaneously observed one-color spectrum with Vuv, and c) the spectrum obtained by subtraction a-b.The subtracted spectrum in c shows the band due to the IR-UV double resonance via J" 8 of the vcH vibration.
Assignments are indicated by solid lines.The notation of the band groups A, B, C, and D are also shown.
simultaneously by the alternative acquisition system, and vR was again fixed to the J" 8 level of the VCH ant vibration in X.Because of the observation in a wider region, intense bands due to Vuv only overlap the double resonance bands.To obtain the spectrum only by the IR-UV double resonance, the one-color spectrum in Figure 4b was subtracted from the spectrum of vr + Vuv in Figure 4a.The subtracted spectrum is shown in Figure 4c.One can see that the bands due to Vuv only are mostly eliminated in the subtracted spectrum.In the subtracted spectrum, three intense band groups marked by B, C and D are found with-1,000 cm differences in energy.The band group B and C are already shown in Figure 3. Furthermore, a weak band group marked by A is found in the region 1,000 cm -1 below the group B. Each band group contains three intense lines which can be assigned to P(8), Q(8) and R(8) from their spacing.
The numbers between groups A---D in the figure show the energy difference between the J 1, K 1 level in each vibronic band calculated from P, Q, R lines.Observed band positions, excess energies from the origin, and calculated energies of J 1, K 1 level energies of the intense bands are summarized in Table 3.Though the difference does not precisely match the known frequency of trans-bending mode V (1,047.55cm-) in ), it is natural to assign them to the progression of vibrational mode V.The gradual increase of the band intensity in going from A to D supports this assignment.The strong appearance of the progression suggests that the origin of the progression A is the fundamental vibration of bu symmetry, i.e.Vcis(in).The vibronic band A has excess energy of 797 cm for J' 1, K 1 level, which is a reasonable frequency for the C-H bending vibration.Therefore, we have concluded that the bands A---D are the progression of V with the false origin of the in-plane cis-bending vibration, Vcis(in) + nV (n 0-3).The pure vibrational energy of each member in the progression is estimated by the same procedure as that used for Vcr ant and is shown in Table 3.The vibrational frequency of vois(in) was obtained to be 785 cm-.The irregular spacing among the members of the progression suggests the strong perturbations in each vibronic level.Since our analysis does not consider the perturbation, the obtained value 785 cm may have large uncertainty.Recently, Ulz et al. also observed the Vcis(in) and Vcis(OUt) vibrational levels in the A state by the near IR-UV double resonance spectroscopy via 3vij ant in the X state. 21The observed positions of the rovibronic bands agree with ours.Their analysis showed perturbations such as Coriolis coupling between vcis(in) and Vcis(OUt and centrifugal distortion, and the authors reported the unperturbed vibrational frequencies of 764.9 + 0.1 cm -1 and 768.3 + 0.2 cm for Vcis(in) and Vcis(OUt).a) Total energy from the zero rovibrational level in .
c) Rovibrational energy of K 1, J level in , obtained from observed P, Q and R lines.
d) Estimated vibrational energy by assuming the typical value (13.1 cm-t) for the rotational constant A.
As described above, all the band groups contain many weak bands.To explain the weak bands, two factors can be taken into account.One is the appearance of the combination bands including Vcis(OUt) which is promoted by Coriolis coupling with Vcis(in).21 Another is the transition from a rotational level of (Vcc + V + Vs) which is the Fermi pair of VCH ant in X.The (vcc + V + Vcis) level lies very close to the Vcn an' vibrational level.Consequently, their rotational structures appear in the same region.The IR laser VIR used in this experiment has just enough resolution to separate the rotational structure of Vc. ant and that of (Vcc + V + Vs).The center frequency of VIR is carefully tuned to the rotational line of Vc. "nt, however a nearby rotational line of (Vcc + V + Vcis) may be weakly excited by the tail of VR.Thus we have to consider four different transitions between two initial levels in X and two final levels in ), of which one is the ) Vcis(in) --X VCH ant transition discussed above.
The weak bands will be assigned to the rotational structures of the three additional transitions.Since the rotational structures of the Vcis(in) vibration and the Coriolis interaction with Vcis(OUt) have been thoroughly analyzed by Ulz et al., 2 we may stop further discussion of the weak bands.
Let us discuss the difference between the IR-UV double resonance spectrum via the fundamental vibration of "VCH ant and that via the combination vibration of 'CH sym "" VCH ant.AS described above, the double resonance spectrum via the VCH sym d-'CH ant combination vibration in X shows only three vibronic bands, VCH ant and its combi- nation vibration with V and Vcc in /.On the other hand, when the 'CH ant fundamental vibration in X is excited by VR, the vibronic bands which contain Vcis(in) appear strongly and the "VCH ant vibronic band becomes very weak in the double resonance spectrum.Since both the IR-UV double resonance spectra are observed in the same region in/, the difference must be explained by the difference in the vibrational character between the VCH ant fundamental vibration and the VCH sym 'CH ant combination vibration excited by 'IR" The combination vibration does not have strong Fermi resonance, therefore only the vibronic bands including single quan- tum of the 'CH ant mode has non zero value of the Franck-Condon factor in transition.On the other hand, the VCH ant fundamental vibrational level is mixed with (Vcc + V + Vcis) by Fermi resonance in the X state, and has a component of vibration.Consequently, the Franck-Condon factor from this level gives non zero value not only for the vibronic bands including VCH ant but also for the vibronic bands including Vcis(in).Therefore, the difference between both spectra is one of the eviden- ces for the strong Fermi resonance of the fundamental VCH ant vibration and for no vibrational mixing of the 'CH sym " 'CH ant combination vibration in the X state.
The IR-UV double resonance LIF spectrum gives us the information on predis- sociation.In our previous work, 2 the absorption spectrum and the fluorescence excitation spectrum of gaseous acetylene were compared.From the sudden and dras- tic decrease in the fluorescence quantum yield, the predissociation threshold to C2H + H in / was obtained to be 46,339-46,673 cm-.The same idea can be applied to the spectrum obtained by the IR-UV double resonance LIF spectroscopy, and the predissociation threshold can be measured by the present spectrum.In the double resonance spectrum via the fundamental vibration, the highest energy band is Vcis(in) + 2V + Vcc, of which the absolute vibronic energy is 46,343 cm-.In the double resonance LIF spectrum via the combination vibration, the highest vibronic band in energy is the 'CH ant q-VCC vibronic band.This band has an absolute vibronic energy of 46,439 cm-1.This energy exceeds the previously obtained lower limit 46,339 cm -1 and is larger than the vibronic energy of Vcis(in) + 2V + vcc observed via VCH ant in i. Therefore the predissociation threshold is newly determined to be 46,439--46,673 cm -1 (133.11+ 0.33 kcal/mol).
IR-UV Double Resonance Spectrum above Predissociation Threshold In the region above the predissociation threshold (46,439--46,673 cm-), the IR-UV double resonance LIF spectroscopy is no longer useful because of low fluorescence quantum yield.On the other hand, the multiphoton ionization spectroscopy becomes available to observe the IR-UV double resonance transition (IR-UV double resonance MPI spectroscopy).The lower limit in energy of the MPI detection is determined by the ionization potential of acetylene (91,950 cm-1).When the IR laser viR is fixed to the VCH sym "k-VCH ant combination vibrational level (6,556.5 cm-1), the lower limit of the detectable region is about 49,500 cm-, where 2Vuv + VR exceeds the ionization potential.Thus the region between the predissociation threshold (--46,500 cm-) and the lower limit of the MPI detection (49,500 cm-) still remains unrevealed.Except for this limitation, the IR-UV double resonance spectroscopy with the MPI detection is an ideal combination.The high sensitivity of MPI enables us to overcome the smaller pumping efficiency to higher overtone, and allows us to observe the weak IR-UV double resonance signal in a supersonic jet.
Figure 5a shows IR-UV double resonance MPI spectrum of jet-cooled acetylene for the VR + VUv energy region of 49,500 cm-1--52,500 cm-1, which is above the predissociation threshold.This spectrum was obtained by exciting the molecule to the J 2 level of VCH sym "t-"VCH ant in X by VIR.The upper trace in the figure shows the spectrum in the presence of VR, and the lower trace in the absence of VR.Both spectra were measured simultaneously by the alternative data acquisition system.
New bands due to VR + Vtjv were found at the positions indicated by A, A', B and C in the figure.The observed positions of these bands are listed in Table 4.These bands are shown in Figure 5b in an expanded scale.As can be seen in Figure 5b, the observed band groups can be classified into the sharp band system A, B and C (FWHM < 1 cm-1) and the broad system A' (FWHM 10 cm-).The difference of the band width suggests a difference in electronic origin or a difference in relaxation rate between the sharp systems and the broad system.In this energy region, three electronic states are predicted by ab initio calculation 3 in addition to the 3[ state; therefore, the electronic state of each band system must be assigned first.
The electronic state can be specified by the transition type.The predicted electronic states are B2 (cis-bent structure), Bu (trans) and 1A2 (cis) in addition to the NAu state.From the X2:g state, the A2 (cis) state is forbidden and can be excluded.
The transitions to the B2 state and to the Bu are parallel transitions.Only the per- pendicular transition is the A-X transition.To analyze the transition type, each band system is observed from various J levels in the VCH sym -" VCH ant vibrational level.
Figure 6 shows the IR-UV double resonance MPI spectra for the region of the band group C obtained by exciting the molecule to the J 0, 1, and 2 levels of VCH sym + Vcr ""t by v. Four bands appear in the spectrum from the J 0 level.In the simultaneously observed MPI spectrum without v,R (lower).v m was fixed to J"= 2 level in the VCH sym q-VCH combination vibration in X.The band groups due to the transition from the combination vibration are indicated by letters A, A', B, and C. b) IR-UV double resonance MPI spectrum around each band group in an expanded scale.
spectrum from J 1, the set of the four bands observed in the spectrum from J 0 is repeated twice with slightly different positions.The set of the four bands is indi- cated by a broken line.In the spectrum from J 2, three sets appear with slightly different positions.This feature can be interpreted to mean that 1) band group C consists of at least four vibronic bands with almost the same rotational constant and 2) the four vibronic bands are perpendicular transitions with the selection rule of AK +1.From the VCH sym "" V'CH ant combination vibration (K 0) in X, only the transition to the K 1 sublevel of the vibronic level in i is possible.The K 1 vibronic sublevel has rotational levels of J > 1, thus only R(0) lines appear for each vibronic level when J 0 of the combination vibration was excited by VIR.Thus the appearance of four R(0) means the existence of four vibronic levels.When the molecule is excited to J 1 of VCH sym "" VCH ant by VIR, Q(1) and R(1) appear for vcHant 7 V (?) Vc + 7 V (?) a) Notations of the band groups shown in Figure 5a.b Total energies from the zexo rovibrational level in X.
c) Rovibrational energies in A measured from the origin of A (42,197.57cm-).
') Assignments of rotational branches P, Q, and R. The branches belong to the same vibronic band indicated by the same numbers of dashes, such as P(2)', Q(2)" and R(2)'.The branches without dash belong to the strongest vibronic band.
e) Estimated vibrational energy by assuming typical rotational constants A 13.1, B 1.08 cm-.
every vibronic level in .I f the four vibronic levels have almost the same rotational constant, four Q(1) lines and four R(1) lines form two sets of bands with similar band spacing.From J 2, each vibronic level shows P(2), Q(2) and R(2) lines, and three sets of bands appear.The parallel transition cannot explain the observed spectra because of its selection rule AK 0. Therefore these bands in group C are concluded to be vibronic bands of the/ state.Similarly, the band groups A and B are also concluded to be the vibronic bands of the state from the analysis of the transition type.It is also concluded that the band groups A, B, and C consist of several vibronic bands, of which the assignments are discussed later.
Figure 7 shows IR-UV double resonance MPI spectra for the region of bands A and A' obtained after exciting the molecule to the (VCH sym "t'-'CH ant) J 0, 2, and 4 levels by VR.The sharp band system A is also the cluster of the vibronic bands of The horizontal scale was drawn by the frequency of Vtv.
the A state similar to C, and the band structure can be well explained by the per- pendicular transition with at least four vibronic bands.On the other hand, the broad band A' shows a different J dependence.As can be seen in the figure, this band appears in the spectrum from J 2 and 4, but disappears from J 0. This behavior means that A' is the transition to the vibronic sublevel of K 2 or 3 which has rotational levels of J > 2 or 3. Since the initial state of VCH sym "k-VCH ant has K 0, this transition has a selection rule of AK 2 or 3.The AK 2 transition due to the deviation from the symmetric top has been found in the/-X band system; however its intensity is very low.Thus the broad band A' cannot be assigned to the vibronic band of/.The natural interpretation is that the band A' is the higher excited state in the region above the ionization potential and is excited by a non-resonant two- photon transition from VCH sym -I" VCH ant in the X state.Since the intensity of band A' shows quadratic dependence for the laser power of Vtv, this higher excited state must be ionized by autoionization.The broad band width is consistent with the fast autoionization.However, such a non-resonant two-photon transition does not usually J=2 43520 43560 43600 Wavenumber, 'UV / cml Figure 7 IR-UV double resonance MPI spectrum of acetylene in a supersonic jet around the band groups A and A' obtained by exciting J" 0, 2, and 4 levels of the VCH sym -l-VCH combination vibration by vR.The horizontal scale was drawn by the frequency of Vuv.
have much intensity.The transition intensity of band A' may be enhanced by the near resonant effect to the sharp band system A at the first photon of Vuv.The sharp band groups A, B and C are assigned to the vibronic bands of the state, and their vibrational energies are about 7,865, 8,810, and 9,515 cm-1, respectively.Each group consists of several vibronic bands.This spectral feature shows that the vibronic levels in this region are strongly mixed with each other.It is straightforward to interpret it by Fermi resonance; each group has a main vibronic band which has a large absorption cross section from X, and its intensity is distributed to nearby dark levels through Fermi resonance.At the present time, it is difficult to assign all the observed bands, therefore, we will discuss the assignments of the main band in each group.
The ordinary ii <---X absorption spectrum shows a long progression of the trans- bending vibration nV (n 0, 1, 2, ...) and its intensity increases with an increase of the vibrational quantum number n.The larger Franck-Condon factor in higher n is due to the geometrical change from linear to trans-bent structure.This intensity distribution for the progression of the mode V is also true in the transition from the VCH sym + VCH "nt vibrational level.In the region below the predissociation threshold, the first and the second members of the progression, VCH ant and VCH ant d-V, have already been found in the double resonance spectrum (see Figure 2).The band in- tensity largely increases in going from the first to the second member.Therefore it is possible to assign the main bands to the members of the progression VCH ant -1-nV, where n 5, 6 and 7 for the main band of the groups A, B and C. The progression nV can start from the 'CH sym -b 'CH ant combination vibration in , because the molecule is excited to the VCH sym + VCH ant level in X by VR.In these assignments, the main bands of A, B, and C correspond to n 2, 3 and 4 in the progression VCH sym + VCH ant + nV.However this assignment has a disadvantage in that the main bands of A, B, and C consist of a lower overtone of the trans-bending vibration V.The lower overtone of the mode V has a smaller Franck-Condon factor.Therefore we tentatively assigned the main bands of A, B and C to be the progression, VCHant + nV (n 5, 6, and 7).
Let us check the validity of this assignment.The difference between A and B is 945 cm -, which is a reasonable frequency of the trans-bending vibration in a higher overtone. 3,32From the tentative assignments VCH ant "1" 5 V for A and VCH ant + 6 V for B, the vibrational energy of a member in the progression is obtained to be (1,049n-9.43n+ 2,857)cm -, where 2,857 cm is the vibrational energy of "VCH ant.Since we cannot specify the main band in the group, the round numbers 7,865 and 8,810 cm -1 are used as the energy of A and B. In spite of the approximate values of the energies, this formula gives a reasonable value 1,039 + 2,857 cm -1 for the vibrational energy of the VCH ant "t-V level (n 1), which is measured to be 3,894 1,037 + 2,857 cm-.This agreement supports the validity of these ten- tative assignments for A and B.
In contrast, the assignment VcHant d 7V may not be appropriate for the main band of C. The difference between B and C corresponds to the frequency of the trans-bending vibration; however, the observed difference 705 cm is too small to assign it to the trans-bending vibration.In actuality, the above formula gives the vibrational energy of the VCH ant -t-7V level to be 9,736cm -(vibronic energy 51,934 cm-1) which is about 200 cm larger than the observed vibrational energy of C (9,515 cm-).If we insist on this assignment, an anharmonicity in higher order or a strong level interaction must be introduced.Thus it may be suggested that the main band of C should be assigned to another combination vibration, such as VCH sym d-),'CH ant -I--VCC d-2V, of which the calculated energy is 9,383 cm -1 without con- sidering anharmonicity.
Whether the main band of C is assigned to VCH ant "t-7V or not, the higher member of the progression is found to disappear.As can be seen in the figure, no band has been found in the region above the band group C. It is highly probable that the progression of the trans-bending vibration appears in the / -X transition of acetylene.Therefore, if the main band of C is not assigned to VCH ant "" 7V, the

Figure 1
Figure 1 Schematic diagram of IR-UV double resonance spectroscopy of acetylene below and above predissociation threshold.
Figure 2 a) IR-UV double resonance LIF spectrum of gaseous acetylene (10 m Torr), b) simultaneously observed one-color LIF spectrum without VR, and c) portions around 46,520 cm Figure 3 a) IR-UV double resonance LIF spectrum of gaseous acetylene (100 m Torr), b) simultaneously observed one-color spectrum without VIR of acetylene.The acetylene molecule was excited to J" 8 of the vcHant vibration by VR.C) The IR-UV double resonance spectrum and d) the one-color spectrum Figure 5 a) IR-UV double resonance MPI spectrum of acetylene in a supersonic jet (upper trace) and

Figure
Figure i IR-UV double resonance MPI spectrum of acetylene in a supersonic jet around the band group C obtained by exciting J" 0, 1, and 2 levels of the VCH sym -b VCH combination vibration by vR.

Table 2
Positions (cm-) and assignments of vibronic bands observed in IR-UV double resonance LIF spectrum of acetylene obtained by exciting P(9) of Vc. ym + vcH in X a Total energy from the zero rovibrational level in .

Table 3
Positions (crn-) and assignments of vibronic bands in IR-UV double resonance LIF spectrum of acetylene obtained by exciting P(9) of VCH in X

Table 4
Positions and assignments of vibronic bands above predissociation threshold observed in sym V in X IR-UV double resonance MPI spectrum of acetylene obtained by exciting R(1) of vCH + CU