THE C=O STRETCHING FREQUENCY IN THE S (xt* -n) STATE OF ACETALDEHYDE AND ITS DEUTERATED DERIVATIVES DETERMINED WITH THE PHOTOFRAGMENT EXCITATION SPECTROSCOPY

The vibrational structure was recorded for the S(rr* n) state of acetaldehyde (CHaCHO) and its deuterated derivatives (CH3CDO, CD3CHO and CD3CDO up to -3500 cm - above the 0-0 bands with the LIF (laser induced fluorescence) and PHOFEX (photo_fragment excitation) techniques. In the PHOFEX spectroscopy, the yield of the formyl radical, HCO(X) or DCO(X), produced in the photodissociation of acetaldehyde is measured agai_nst the excitation frequency. The yield was determined by monitoring one of the rotational lines in the B <--- Y( transition of the formyl radical by the LIF technique. This is the first measurement of the vibronic structure of acetaldehydes in supersonic jets above the dissociation threshold where they are non-fluorescent. The results have made it possible to locate v4 (the C=O stretching) and 2v bands unequivocally; the v fundamentals at 1217, 1189, 1210 and 1178 cm - above the 0-0 band for CH3CHO, CH3CDO, CD3CHO and CD3CDO, respectively. The corresponding overtone (2v4) bands are observed, respectively, at 2414, 2350, 2399 and 2334cm -I. All the vibronic bands newly observed in the PHOFEX spectra are found to be interpreted as the combinations of v4 and 2v with the modes appearing in the lower frequency (< 1200 cm -) region, which consist of v0 (the C--C--O in-plane bending), v4 (the C=O out-of-plane wagging) and v5 (the methyl torsion).


INTRODUCTION
The spectroscopy and dynamics of acetaldehyde in the gas phase have been studied extensively, since it is considered to be one of the typical carbonyl compounds whose photochemical behaviour attracts profound interest, Recently, we have reported detailed studies on the photodissociation of jet-cooled acetaldehyde in the lowest excited singlet electronic (S) state, [2][3][4] where the rotational and vibrational excitation of the product, HCO, have been determined as a function of dissociating photon 250 TAKUMI KONO et al.
energy. The threshold for the appearance of HCO has also been determined. The rotational excitation of HCO was found to be determined solely by the energy released at the exit channel below the barrier, while the vibrational excitation is determined by the excess energy above the reaction barrier. 4 The vibronic structure in the S(rr* n) state of acetaldehyde has long attracted interests in relation to the structural change accompanying the rr* (---n transition and also to possible mode selective dynamics. In spite of intensive investigations, the vibronic assignment is still incomplete; in the early assignments performed for the electronic-absorption or LIF (laser induced fluorescence) spectra measured at room temperature, 6-8 the 0-0 band was misassigned because of the spectral congestion due to the hot species. The location of the 0-0 band was definitely fixed later in the highly resolved LIF-excitation spectra measured for jet-cooled acetaldehyde. It has been found that most of the vibronic bands appearing up to 1000 cm -1 from the 0-0 band are assigned to the fundamental, overtone and combination bands of the Vo (the in-plane CCO bending), VI4 (the C--O out-of-plane wagging) and v15 (the methyl torsion) modes. 5,9-The frequency of the v4 (the C--O stretching) mode in S is of particular interest, since its frequency is expected to decrease drastically from the ground-state value because of the occupation of the rr* bond. It is also expected that the v4 mode is Franck-Condon active because of the change in the C--O bond length upon the electronic excitation. 8 The electronic excitation of CH3CHO above 0 + 1420 cm -! causes the dissociation into HCO and CH3, which quenches the fluorescence. Therefore, only the vibronic structure below -1800 cm can be measured by the LIF technique. This is one of the reasons which make the definite assignment of the C--O stretching band difficult.
Recently, we found the PHOFEX (photofragment excitation) spectroscopy to be useful in obtaining information on the vibronic structure of acetaldehyde above the dissociation threshold. In this scheme, the formyl radical (HCO or DCO) in the ground electronic state, which is one of the dissociation products, is monitored by the LIF technique with a delayed_ probe pulse. The formyl radical can be probed sensitively by employing its B <--X transition. 4-6 Excitation spectra of acetaldehyde are obtained by measuring the yield of the formyl radical against the pump frequency. In this paper, we present the PHOFEX spectra of acetaldehyde and the deuterated derivatives (CH3CDO CD3CHO and CD3CDO in supersonic jets up to --3500 cm above the 0-0 band. The spectra are analyzed together with the LIF-excitation spectra with particular emphasis on the determination of the vibronic band due to the v4 mode. THE C=O STRETCHING FREQUENCY IN THE $1(* -n) 251

EXPERIMENTAL
The LIF-excitation spectra of acetaldehyde were measured with the frequencydoubled output of a dye laser (Quantel: TDL-50, 0.5--1 mJ/pulse, line width: 0.2--0.5 cm-1) pumped by the second harmonics of a Nd:YAG laser (Quantel: YG571C). The same laser operated with the higher output power (1.5-3 mJ/pulse) was used as the pump laser for the PHOFEX measurements. The frequency of the laser was calibrated with a "see-through" hollow cathode lamp (Hamamatsu: L2783-13-NE-AL). The probe source for the PHOFEX measurements was an excimer-laser-pumped dye laser (Lumonics: HE-420-SM-B and Lambda Physik: FL3002,-0.8 mJ/pulse, line width: --0.2 cm-). The probe frequency was set at one of the rotational lines of the formyl radical (HCO or DCO) in the 0-0 band of the 1 +--J transition.
The laser beams were aligned almost collinearly, and focused onto the molecular beam 16 mm downstream from the nozzle exit. Total emission from acetaldehyde or the formyl radical was collected onto a photomultiplier (Hamamatsu: R955), whose output was integrated by a boxcar averaging system (Stanford Research Systems: SR235, SR240, SR243 and SR250). In the LIF measurements, the gate (100 ns) of the boxcar was opened 50 ns after the excitation pulse. In the PHOFEX measurements, the delay between the pump and probe pulses was 1/s with the 70 ns gate opened simultaneously with the probe pulse. In the PHOFEX measurements, a band-path filter (HOYA: U-330) was used in front of the photomultiplier to eliminate the scattering of the laser pulses.
Helium containing ca. 5% acetaldehyde was expanded into a vacuum chamber from a conventional pulsed valve with a 0.8-mm-diameter orifice. Acetaldehydes [CH3CHO (Merck, > 99.5%), CH3CDO (MSD isotope, 97 atom % D), CD3CHO (MSD isotope, 98.3 atom % D) and CD3CDO (Aldrich, 99+ atom % D)] were used without further purification. The stagnation pressure of the gas mixture was 2.2 atm. The background pressure in the chamber was held below 10 -4 Torr during the measurements. Figure 1 shows the (a) LIF-excitation and (b) PHOFEX spectra of CH3CHO in a supersonic jet obtained by pumping the Sl(rr* n, .,) state. The ordinate shows the frequency shift from the 0-0 band at 29769 cm -l (= 0). These spectra are not calibrated for the pump power, which varied by a factor of about 2. The band intensities can, therefore, be discussed only qualitatively. The LIF-excitation spectrum is in agreement with the spectra reported previously. TM No hot band is observed in the energy region lower than the 0-0 band.  shows the frequency shift from the 0-0 band at 29769 cm-.

RESULTS AND DISCUSSION
The fluorescence from CH3CHO is observed only for the excitation frequencies lower than 0 + 1835 cm-, above which it disappears because of the opening of the dissociation channel to HCO and CH 3. Therefore, vibronic structure of acetaldehyde in the higher frequency region cannot be measured by the LIF technique. A few spectroscopic techniques have been proposed to measure the excitation spectra of the non-fluorescent compounds. Among them, the hole-burning spectroscopy (pumpprobe absorption depletion spectroscopy) 7-9 and the REMPI (resonance enhanced multiphoton ionization) technique 2,2 seem to be difficult to be applied to acetaldehyde because of small cross sections of the vibronic transitions and the fast intersystem crossing for the higher vibrational levels in S. On the other hand, the PHOFEX spectroscopy is applicable, in which one of the dissociation produ_cts, formyl radical, is probed sensitively with the LIF technique employing the B <--transition.2,4, 6 The PHOFEX spectrum in Figure 1 was measured by s_etting the probe frequency at a rotational line in the 0-0 band of the HCO I <---X transition. The rotational line employed is indicated in the LIF-excitation spectrum of HCO shown in Figure  2a. The bands appearing in the PHOFEX spectrum below the onset of the fluorescence (< 00 + 1835 cm-) are at least partly due to the long tails of fluorescence which could not be removed completely even with the delay of 1 ts between the pump and probe pulses. This, however, would not bring about any serious difficulty in the vibronic assignments. shown in Figure 2b, where the rotational line used in the PHOFEX measurements is indicated. Inspection of these spectra gives us an insight that almost all the vibronic band observed for these four isotope species can be assigned to the repetition of the same vibronic structure starting at 00 , 40 and 420.
The v4 mode is expected to be Franck-Condon active in the excitation spectrum of acetaldehyde. TM It has been pointed out that one of the bands around 00 + 1200 cm should correspond to the v4 mode. ''' To facilitate the assignment, the CD3CDO spectra are rearranged as illustrated in Figure 6, where (a) is the excitation spectrum synthesized from the LIF-excitation and PHOFEX spectra shown in Figure 5. Figure 6b is the same as the lower energy part (< 2000 cm-') of (a) shifted to the higher energy by 1178 cm-'. In (b), the bands assigned to those belonging to the progression starting from the 00 are shaded. The corresponding 254 TAKUMI KONO et al. bands in (a) are shaded to facilitate the comparison. It is seen that the shaded bands in (a) can be assigned perfectly as the repetition of the bands starting at 0o [shaded in (b) below 1600 cm-1]. The non-shaded bands in (a) above 2300 cm -l corresponds well to (c), which is again the repetition of (a) starting at 2334 cm-1. We assigned the 1178 and 2334 cm -1 bands to 40 and 402, respectively. It is to be noted that the assignment of 4 and 402 is unequivocal, since the characteristic feature in the 0--500 cm -1 region due to the VI5 methyl torsional mode is repeated from the 4 and 40 bands; no reasonable assignment of all the observed bands is possible if one shifts the frequency of 4 or 4. Similar analyses are given in Figures 7-9   The fundamental frequencies of about 1200 cm -1 for the C--O stretching in S may be compared with the corresponding values of 1743 cm for CHCHO in the ground electronic state. A large decrease of frequency is due to the increased antibonding character of the C--O rr bonding in the excited state.
The frequency of the C--O stretching depends more strongly on the deuteration of the x hydrogen than that of the methyl group. The frequency shifts on the methyl deuteration (CHCHO to CDCHO, and CHCDO to CDCDO) are estimated to be about 1.7% on the basis of the calculation in which CHCH or CHCD is considered to be a mass point. The observed shifts (0.6 and 0.9%, respectively) are smaller than this estimation. A similar estimation gives the shifts of 0.7 and 0.6%, respectively, for the o deuteration (CHCHO to CH3CDO, and CDCHO to CD3CDO), while the observed shifts (2.3 and 2.6%, respectively) are much higher, suggesting appreciable coupling of the C--O stretching with the HCO bending and/or the CH stretching.
Noble et al. 9, and Baba et al. have previously concluded that most of the bands observed in the lower frequency region (< 00 + 1000 cm-) in the LIF-excitation spectrum of acetaldehyde can be assigned to the fundamental, overtone and combination bands of the v0, v4 and v modes. The methyl torsion (v) shows a characteristic feature due to the hindered rotation, which tends to disappear above -500 cm corresponding to the barrier for the internal rotation. The congested structure appearing in the 500--1200 cm region should be assigned to the combinations of Vo and/or '14 with v.
The v4 (the C--O out-of-plane wagging) mode has a double-minimum potential with a barrier of 540 cm for CH3CHO. This is because the molecule deforms out-of-plane in the S state. The vibronic progression due to v4 would, therefore, exhibit a complicated pattern around the barrier top. An additional complication arises in this molecule, since the group-theoretical consideration is not helpful; the vibronic transitions to the states which are symmetric with respect to the molecular plane are symmetry allowed, but have very weak intensity. On the other hand, the antisymmetric vibrational states can be observed due to the vibronic coupling with higher-energy allowed electronic transitions. The intensities arising from both sources are comparable with each other in this case. This situation, together with the existence of lower barriers both for the v4 and v5 modes, have made the previous assignments difficult in the 0-1200 cm region.
In the present study, we have succeeded in obtaining well resolved spectra for four isotope species of acetaldehyde to much higher energy than previously reported and also in separating the vibronic progression starting at 0 unequivocally from those starting at 46 and 4. Now we are in the position that we could reexamine the assignment in the 0-1200 cm region thoroughly, since we know which bands belong to the progression starting at 00 and we can compare the effects of deuteration at methyland o-position completely. The assignment is being performed in our laboratory and will be published in near future.