THRESHOLD IONIZATION SPECTROSCOPY OF THE LOW FREQUENCY VIBRATIONAL MODES OF STYRENE AND TRANS-STILBENE CATIONS

High resolution photoelectron spectra are reported for styrene and stilbene with an emphasis on iden- tifying the low frequency modes which are important in the phenyl-vinyl torsional motion. In the styrene cation three low frequency out-of-plane vibrations are measured, v42, v41 and v40 as well as a number of higher frequency in plane modes. Generally the low frequency modes do not appear with significant intensity but by pumping the appropriate modes in the S intermediate state the intensity for these modes in the threshold ionization spectrum is quite good. In the stilbene cation the analogous phenyl-vinyl torsion, v37 is also measured as well as the related mode ’36" The ethylene torsional mode in stilbene is not observed. In both molecules the observed frequencies are used to relate the electronic structure of the cation to the S and So states of the molecule.


I INTRODUCTION
Styrene and stilbene represent model compounds in the investigation of the effect of electronic excitation on conjugation. In each molecular system there are two important out-of-plane torsional motions, the phenyl-vinyl torsion (Ceth--Cph) and the torsion about the ethylene double bond. In styrene 1-4 the focus of interest has been the phenyl-vinyl torsional coordinate in regards to the equilibrium geometry, the potential about the local minimum and the barrier height for rotation. In stilbene the largest concern is the torsional barrier between the cis and trans form which is dependent on rr bonding in the vinyl group. In the ground state, this barrier at 90 is high,--16,000 cm-. However, photoisomerization takes place on this torsional coordinate in the excited state where the potential barrier for isomerization is much lower, -1,400 cm-. The low excited state barrier is due to electronic state interactions. In addition to the isomerization torsional coordinate experiments have also revealed much about the phenyl-vinyl torsion similar to the motion in styrene. It is this torsion which is observed in the photoelectron spectrum and is the focus of this work. In both molecules the low frequency torsional vibrational mode involving the twisting of the ethenyl group about the single bond serves as an excellent probe of the interaction between the aromatic and ethenyl rr electrons and the extent of electronic coupling to the torsional potential. The effect of electronic excitation on conjugation in these compounds can be examined by observing the change in vibrational frequency upon electronic excitation as well as possible mode mixing in the coupled low frequency motions. Such studies, employing So to S excitation have been performed on a number of molecules including styrene TM and stilbene. 7-Herein we extend such studies to the cation ground state. This allows a further probe of the influence of the highest lying molecular orbitals on the torsional potential.

Styrene
The spectroscopy of the ground and first excited singlet state of styrene has been carefully detailed in a number of studies. [2][3][4] Planar styrene is of Cs symmetry and thus normal modes fall into two groups, a' and a". The a' vibrations, v to "V29 are symmetry allowed for the electronic transitions between the ground state, So (A'), and the excited state, S (A'), while a" vibrations, v30 to v42 can appear only as even overtones or in totally symmetric combination bands. The So --S transition involves excitation of rr electrons to arr* antibonding orbital. The majority of the intense bands that appear in the excitation spectrum are benzene like modes which do not include the vinyl substituent, indicating that the So --+ S electronic excitation takes place largely on the ring.
Information about the nature of the cation potential energy surface can provide important details about the ethenyl-benzene system conjugation and the associated molecular orbitals. Dyke et al. 12 performed a ZEKE-PES study on styrene revealing some of the vibrational frequencies in the cation but did not observe the bands associated with the ethenyl torsion. The results reported here include vibrational analysis and assignments using ZEKE-PES of cation vibrational bands which are sensitive to the ethenyl torsional potential and which provide information about the nature of cation potential energy surface. This study concentrates on the changes in the structure of styrene upon electronic excitation as manifest in the appearance of vibronic bands in the cation spectrum. These changes are apparent in the comparison of vibrational frequencies, couplings, and intensities in different states and have connections to the description of the torsional barrier. Detailed studies have been done on the ground and S state including vibrational analysis 2-4 and calculations. 3 Cation spectra obtained by ZEKE PES provides information about rr bonding in the cation after removal of the HOMO electron.

Stilbene
The low frequency modes of stilbene have received great attention because of the interest in the cis-trans isomerization process in both the ground and excited states.
Planar 4 trans-stilbene has C2 symmetry with optically active modes of a symmetry.
The primary torsional mode under consideration here is v37, Figure 1, which is of  respectively. The extremely low frequency mode is very anharmonic in the ground state but is much more harmonic in the $1 state as the frequency increase significantly. Mode 36 is primarily an out-ofplane phenyl bending motion 9,1 and is not observed in emission from the origin of S. However, it is observed in emission from the 372 excited state indicating significant mode mixing of 36 and 37 upon excitation. This is confirmed by observation of the 3637 combination band in the excitation spectrum.
Herein we report the ZEKE spectrum of stilbene for the first time and again focus on the nature of the low frequency modes in the cation.

II EXPERIMENT
The threshold ionization photoelectron spectroscopy, also known as pulsed field ionization (PFI) and zero electron kinetic energy (ZEKE) spectroscopy, 5 is performed as described in detail in a previous publication. 6 The spectroscopy is a two color pump-probe process in which the pump laser is tuned to a vibronic resonance in the S electronic state and the probe is then tuned to further excite the molecules to the ionization threshold and above. Just below each cation resonance is a dense manifold of high n Rydberg states. It is these Rydberg states which are the final state in the pulse excitation sequence. The Rydberg states are created in an initially field free region but are then field ionized by a-30 V/cm pulse which is applied approximately 1 psec after their creation. The delay time allows all prompt electrons (non-resonant) to leave the focal volume and only the Rydberg derived electrons are then detected. The spectral resolution is --6-10 cm -1 for our spectra and is determined primarily by the extraction pulse voltage. A number of studies have indicated that the intensity of the observed peaks is primarily determined by Franck-Condon factor although some evidence exists to suggest this is not always the case.
The experiments are performed in a pulsed supersonic beam apparatus which consists of two differentially pumped chambers. The second chamber houses the photoelectron spectrometer which detects the electrons perpendicular to the laser and molecular beam directions. The pulsed valve (General Valve Inc.) is mounted in a heated flange and also contains a separately heated sample reservoir in close proximity to the nozzle. Stilbene was heated to 100C (-10 Torr vapor pressure) and the valve itself was maintained at a slightly greater temperature. Styrene was contained at 0C (--5 Torr vapor pressure) in an external sample container through which the He carrier gas (30 psig) was bubbled.
Two laser systems were used for the studies presented. Most experiments were performed with a 20 Hz frequency doubled ND YAG laser (Continuum NY-61) pumping two high resolution (.04 cm-) grazing incidence dye lasers (Lumonics HD 500). The output of each dye laser is frequency doubled and one is used as the pump and the other the probe. The pump and probe lasers were equipped with autotracking frequency doubling as required. The second laser is a two color pulse amplified picosecond system which has been describe previously. 17 Although the spectral resolution of this system is significantly lower (3-5 cm-1) it is comparable to the width of the threshold ionization peaks and therefore does not compromise the experimental resolution to a great degree.
For each molecule a series of photoelectron spectra are scanned, each with a dif-  In fact a demonstration of the sensitivity of ZEKE spectroscopy is that a high quality ZEKE spectrum can be obtained by pumping the 4202 transition which is only 1% of the intensity of the electronic origin. Figure 3 shows the ZEKE spectra obtained by pumping each of these bands in S1. It is expected that the strongest feature in each cation spectrum is the Av 0 transition. This is confirmed by scanning the ZEKE spectrum from the $1 origin which is dominated by a large peak (Av 0) which appears at the expected adiabatic ionization threshold. Given an So to $1 transition energy of 34,758 cm -t the adiabatic ionization potential is determined to be 68,267 cm-, in close agreement with the reported work of Dyke et al. 12 Once the Av 0 propensity is established the low frequency modes can be assigned by identifying the strongest 1+ 421+ and 40 + transitions in each of the above mentioned spectra as 42 +, 41 41 + respectively. Table 1 summarizes the So, $1 and cation energies for these combination bands. Unfortunately the important v38 mode is not observed in the $1 spectrum and so its value could not be determined in the cation. In addition to the a" modes a number of higher frequency a' modes were investigated, specifically modes 23, 24, 25, 27, 28 and 29. The frequencies of all the modes measured are summarized in Table 2 including the values for the So, S and cation ground state.
Note that mode 42 is attained by assuming the observed cation overtone is harmonic and taking half of the frequency of the 422+ band. This is often a poor approximation for low frequency, large amplitude motions but v42 is quite harmonic in S12 and  since the cation frequency is close to that in S1 it is likely to also be harmonic in the cation. Once v42 is established then v41 and v40 are determined from the observed combination bands. The vibrational bands that are most sensitive to the conjugation in the vinyl substituent are the a" modes v38 and v42 both of which are depicted in Figure 2 This can be compared with the v38 ground state frequency of 640 cm showing that the C-C, bond takes on more double bond character in the excited state but is still primarily a single bond.
The ZEKE spectroscopy reveals that the cation frequency of mode 42 is 184 cm-, representing little change from the excited state frequency of 185 cm-. Removing the rr* electron appears not to change the bonding between C-C, and thus the conjugation through the vinyl group. Mode 41 has a frequency of 97 cm in the cation, which is exactly the value in S but significantly less than the 199 cm in the ground state. Again, the $1 and cation frequencies nearly match. More typical is a correlation between the ground state frequency and that of the ground state of the cation as in the case of 9,10-dihydrophenanthrene. 17 Mode 41 is lower in frequency upon excitation, perhaps related to the increased flexibility in the ring due to the antibonding rr electron in S1 and the removal of this electron in the cation. Mode 40 is found to have a cation frequency of 354 cm-1, which is an increase from the excited state frequency of 252 cm-1. This mode is localized on the benzene ring and, as such, is associated with benzene mode 16a.
The ZEKE spectra arising from totally symmetric vibrations, a', also are examined.
Modes 28, 24, and 18 are related to benzene modes 6a, 12, and 18a respectively. The excited state frequencies of these benzene-like modes in styrene correlate with those of alkylbenzenes upon comparison with work by Hopkins et al. 8 This indicates that the vinyl group represents only a slight perturbation to the ring system. Comparison with stilbene is discussed in a later section. The cation spectra reveal some changes in these frequencies. Mode 27 and 29 both involve substituent bends and should thus be sensitive to the changes in the vinyl group. Both modes decrease in frequency upon excitation and in the cation their frequencies are the same or higher than in So. This data and the fact that mode 40, partially a ring out of plane mode, follows the same trend indicates that the substituent has more flexibility out of plane in $1 than in the ground and cation ground states.
Electronic structure calculations of Hemley et a113 are particularly useful in assessing the effect of electronic excitation on the low frequency torsional modes. They used PPP-CI and CNDO/S-CI calculations to evaluate the electronic energy, molecular structure and molecular orbital configuration interaction in each electronic state. These calculations show that the effect of So to $1 excitation is to shorten the Ceth--fph bond. This effect is attributed to a mixed molecular orbital excitation which contains a significant contribution from a nonbonding-to-bonding transition and an antibonding-to-nonbonding transition with respect to the Ceth--Cph bond. This increased Ceth--Cph bond strength in S is reflected in the increase in frequency of v42 from 38 cm in So to 185 cmin S. As mentioned above it was also shown that extensive low frequency mode mixing occurs as a result of the S excitation.
The cation electronic structure can be evaluated by applying Koopman's theorem and looking at the change in bonding due to removal of an electron in the highest lying molecular orbital. In many aromatic molecules characterized by rr to rr* excitations the cation frequencies often are between those of So and S1 because the anitbonding electron is removed upon ionization. In styrene the low frequency modes are quite similar to the S state. This suggests that the highest lying electron is primarily nonbonding with respect to the Ceth--Cph bond, and thus its removal has little effect on the bond strength and resulting vibrational frequencies. Thus one would expect that the torsional potential of the styrene cation is very similar to the S1 state of the molecule. The nonbonding nature of the excited state (with respect to Cet-Cp) is consistent with the leading configuration to S calculated by Hemley et al. 3 Stilbene ZEKE spectra were obtained from the following four different S vibronic pump transitions; 0 , 36'37', 3T, 25', 24'. Again, as in the case of styrene, the origin ZEKE spectrum exhibits a strong propensity for the Av 0 transition which should also apply to the other transitions studied. The measured ionization potential is 61,756 cm which is in good agreement with the 61,750 cm-' value determined by Suzuki et al using a Rydberg series extrapolation. Several of the ZEKE spectra are shown in Figure 4. The data for stilbene was taken with the picosecond laser system described above and thus the relative and absolute band positions are subject to greater uncertainty. The cation vibrational mode frequencies have a conservative error of +5 cm-.
Applying the Av 0 propensity allows the assignment of the low frequency modes in the cation ground state. Using the 372 pump transition yields a value for 372+ of 78 cm as compared to 95 cmfor 372 in the S state. Since 37 + is not observed directly in the ion spectrum its frequency is estimated by taking half of the overtone to yield a value of 39 cm-. This is a reasonable approximation because v37 appears harmonic in S and is not drastically different in frequency in the cation. Once v37 is established the value of v36 can be determined from pumping the S 3637 band.
The value of 361+ is thus determined to be 42 cm -1 in the ion which is the same value as assigned to $1 so it undergoes no significant shift upon ionization. A summary of the cation frequencies obtained for stilbene is given in Table 3. As in the case of styrene the phenyl torsional potential in the stilbene cation, as evidenced by the frequency of v37, is similar to the S excited state. One can interpret this again as primarily due to a loss of antibonding character in the Cph-Ce bond on promotion of the HOMO electron from So to S without much bonding contribution in the LUMO. Therefore upon ionization loss of an essentially non-bonding LUMO electron causes little change in the Cph-Ce bond strength or resulting torsional frequency. In regards to this torsional motion the behavior of styrene and stilbene are actually quite similar. The difference in torsional mode frequency is dominated by the contribution of the change in the reduced moment of inertia between the two molecules. Also the behavior upon electronic excitation is quite similar with the frequencies increasing by a factor of 4.9 in S for styrene and a factor of 5.9 for stilbene. The cation potentials in both molecules are very similar to those in S.
Therefore it is clear that the electronic structure of these molecules is quite similar with regards to the Cp-Ce conjugation.
In stilbene v25 is a prominently observed ag vibration in the So to S spectrum and consists of an in plane Cphenyl-Ceth-Cet bend. The vibrational activity is ascribed to an increase in the bond angle upon excitation of approximately 1.3. v25 is clearly observed in the cation spectrum when pumping the 251 transition and is assigned a frequency of 182 cm -1 as compared to 202 cm in So and 198 cmin $1. v25 is also observed prominently in the ZEKE spectrum obtained when pumping the S origin which suggests that the bond angle also changes upon ionization. Although the bond angle may be changing the small frequency change upon electronic excitation indicates that the bending flexibility of the vinyl bridge is changing very little.
IV SUMMARY ZEKE photoelectron spectroscopy has been used to study the low frequency modes of styrene and stilbene. Although the spectral intensity of these modes is quite low in the So --$1 transition it is shown that these states can still be used as intermediate vibrations in the ZEKE process and thus reveal the frequencies of these modes in the cation. The phenyl-vinyl torsional potential in both molecules is observed to change only slightly upon excitation from S to the cation, even though larger changes are observed for the So -$1 transition. Particularly in the case of styrene these vibrational changes can be compared to calculations which assess the change in bonding between Ceth--Cph upon ionization. Since the highest lying electron in the S configuration is non-bonding its removal does not have a great influence on the Ceth--Cph torsional potential.

V ACKNOWLEDGMENT
We gratefully acknowledge the NSF for support of this research.