J. Phys. Chem. 1995,99, 2583-2588
2583
Two-Color Laser Photoelectron Spectroscopy of Electronically Excited Cations: Vibrationally Resolved Spectrum of the 2A2 State of Aniline+ Byungjoo Kim and Peter M. Weber* Department of Chemistry, Brown University, Providence, Rhode Island 02912 Received: July 27, 1994; In Final Form: October 24, 1994@
Two-color ionization photoelectron spectra of aniline via the neutral molecule vibronic states SI(OOo) and Si(6a'o) are presented. The intermediate vibrational states of the SI manifold are prepared by a picosecond laser pulse near 290 nm, and the subsequent ionization is induced by a time delayed 200 nm laser pulse. Mixing of molecular orbitals leads to an unexpected optical transition from the Sl('B2) state to the electronically excited 2A2 state of the cation. The ionization potential of the latter state is found to be 9.031 f 0.010 eV, slightly higher than the He I photoelectron spectroscopy value reported in the literature. Comparison of the spectra obtained via the two different vibronic resonances allows us to assign a 65 meV progression of vibrational peaks in the cation 2A2 spectrum to the 6a mode. The partial ionization cross section to the excited ion 2A2 state is measured to be 7 f 2 % relative to the cross section to the ionic ground state. INDO/l calculations suggest that the optical transition to the 2A2 state is due to initial state configuration interaction, i.e., a mixing of the SI state with higher electronic states in the neutral molecule.
Introduction Vibrationally resolved spectra of electronically excited states of molecular cations can provide valuable insights into the character of molecular orbitals of both the ion and the neutral molecule.' Techniques to observe excited ion states include fluorescence spectroscopy of cations that are generated in a plasma2 or by electron impact i~nization.~ In order to increase the resolution and reduce the congestion of the spectra, free jet expansion techniques have recently been adapted for ion ~pectroscopy.~ The halobenzenes were found to be particularly favorable systems as their excited ion states are sufficiently long lived to ensure photoemission with a reasonable quantum ~ i e l d . ~Unfortunately, -~ many polyatomic molecules do not offer large fluorescence quantum yields, so that fluorescence spectroscopy has limited applicability. While He I photoelectron spectro~copy~.~ can be applied to all molecules, precise investigations of the electronic structure of the excited ion states are difficult because of the poor resolution of most electron spectrometers and because of the vibrational congestion associated with the frequently used effusive beams. Furthermore, the assignment of only partially resolved vibrational structure is limited by the lack of corroborating evidence from separate experiments. In an attempt to develop experimental tools that are independent of fluorescence emission, a mass-selected ion dip technique was recently developed.8 A molecular ion is prepared in a specific vibrational level of the ionic ground state by resonance enhanced multiphoton ionization via selected vibronic levels of the neutral molecule. A tunable laser pulse then excites the ions to their electronically excited states which, if the ion dissociates on a time scale shorter than the radiative or nonradiative conversion to its ground state, leads to a dip in the intensity of the ion current at the parent mass. While the ion dip technique is to some extent complementary to the fluorescence techniques, it remains desirable to develop a molecular beam based technique that is generally applicable and that provides a way to map vibrational spectra of excited ion states onto the well studied potential energy surfaces of the neutral molecule. @
Abstract published in Advance ACS Abstracts, February 1, 1995
0022-365419512099-2583$09.00/0
A technique that may closely match these requirements is resonance-enhanced two-photon ionization photoelectron spectroscopy, which has been applied to study the vibrational structure of polyatomic molecular cations in their respective ground electronic s t a t e ~ . ~ -By l ~ selecting different vibrational resonances of the SI or higher electronic statesI3in the ionization step and observing the resulting vibrational progressions in the ion spectrum, one establishes a Franck-Condon relationship between the electronic surfaces of the neutral molecule and the ground electronic state surface of the cation. This relationship can be applied to unravel the assignments of the ion vibrations provided the vibrational spectroscopy of the intermediate state is known or to infer details about the intermediate resonance if the cation state is well characterized. The latter experiments are amenable to study both the spectroscopy and dynamics of very short lived, nonfluorescent molecular resonance^.^^-^' The spectroscopic assignments are dramatically simplified by seeding the molecules in a molecular beam to reduce the vibrational congestion. Vibrational structure is then well resolved for medium size molecules such as benzene and and even large polycyclic molecules such as ~henanthrene.'~ The abundant applications of two-photon ionization photoelectron spectroscopy in ground electronic state spectroscopy of cations suggest that the technique may be extended to excited cation states. The development of such a technology is presently limited by the difficulty to generate very short wavelength laser pulses as required to access the excited ion states and the realization that transitions from excited states of the neutral molecule to excited cation states may be subject to restrictive selection rules. In the present paper we demonstrate that couplings of electronic states in both the ion or the neutral molecule can lead to a breakdown of the simple one-electron, one-photon description of the ionization transition, so that excited ionic states can indeed be accessed from excited neutral states. The powerful methodology of multiphoton ionization photoelectron spectroscopy can therefore be applied to study the vibrational structure of electronically excited molecular cations. The system we studied is aniline, CsHsNH2, which is excited to certain vibrational levels of the SI state by a laser pulse of 0 1995 American Chemical Society
Kim and Weber
2584 J. Phys. Chem., Vol. 99, No. 9, 1995 293.8 or 289.7 nm wavelength. The photoelectron spectrum of the molecule is obtained after ionization from the SI state with a 200 nm laser pulse. The 200 nm photon reaches well above the first excited electronic state of the cation, a state that is assigned to 2A2 symmetry based on the molecular orbital associated with the ionization .22-25 While the ground electronic state, So('Al), and the first excited state, Sl('B2), of the neutral molecule as well as the ground electronic state, (2B~),of the molecular cation have been well studied, little information about the excited ion state is available. Aniline in the ground electronic state is known to have a nonplanar structure with the hydrogen atoms of the amino group bent out of the aromatic ring plane by 42°.26,27When aniline is excited to the SI state, this bending angle vanishes as the hydrogens move to the plane of the aromatic ring.28 The delocalization of the amino nitrogen's lone pair electrons over the aromatic ring causes the molecular cation in its ground electronic state to have a planar The same kind of delocalization might be structure as expected to lead to a planar structure for the molecular cation in its first excited electronic state. Our spectra show wellresolved vibrational structure corresponding to the first excited electronic state, allowing us to discuss the vibrational structure and the geometry of this state. We point out that this discussion is based on Varsanyi's numbering system for the normal modes of aniline.30 The symmetry labels of vibrational modes and electronic states are based on the C2,, point group of the planar molecule. Experimental Details The supersonic expansion, time-of-flight photoelectron spectrometer, and its detection system have been described previ~ u s l y , ' * -and ~ ' we only outline those changes that pertain to the present experiment. Most importantly, we adapted the laser system to the requirements of two-color photoelectron spectroscopy. The frequency-doubled output of a mode-locked Nd: YAG laser (Spectra Physics 3800LS) is now split to synchronously pump two cavity-dumped dye lasers (Spectra Physics 3500). The outputs of the dye lasers consist of trains of 7 ps pulses at a repetition rate of 800 kHz. The intermediate vibronic level of aniline is prepared by tuning the output of the first dye laser, which is frequency doubled in a BBO crystal, to the SI resonance frequency of aniline. Within the wavelength range accessible by the second harmonic are two resonances, identified as S1(O0o) and S1(6a'o), based on the works of Brand et Smith et and Chernoff and Rice.34 The second dye laser, which is pumped by the time-delayed green light of the YAG laser, is frequency-tripled in two consecutive mixing steps.35 In a first step the second harmonic is generated using a temperature-tuned, 20 mm long ADA crystal. The polarization of the remaining fundamental is rotated by 90" using a half wave plate that acts as a full wave plate for the second harmonic. Both fundamental and the second harmonic are focused into a 80" cut BBO crystal using astigmatically compensated reflective optics to obtain a UV beam tunable around 200 f 2 nm with a power of up to 1 mW at a repetition rate of 800 kHz. The near- and far-UV beams are superimposed using a dichroic mirror. The time interval between the two laser pulses is adjusted by controlling the delay time of the 532 nm light that pumps the second dye laser and by matching the timing of the two dye laser cavity dumper drivers. For the present experiments the 200 nm laser pulse intersects the molecular beam within 200 ps after the excitation laser pulse. As detailed in a separate paper,36the intersystem crossing dynamics of aniline in its S I electronic state is on a time scale much longer than this delay time, so that the spectra can be analyzed using Franck-Condon factor arguments.
TABLE 1: Observed Vibrational Peaks in the 2A2 State upon Ionization via the SI Origin
Peak Dosition (meV).
intensity exptb
calcn'
0
55
63
65 104
100
130 170 194
100 32 70 38 53
232
26
258
31
326
assignt origin (6a"l") 6a' 1' 6a2 6a' 1
82
41
6a3 6a21'
21
6a4
6a5
8
Peak position relative to the vibrationless ion peak at 9.031 meV. Averaged over several measurements. Based on a displacement of the equilibrium position of the 6a mode in the 'A2 state of 0.6 x g"* cm from that in SI state.
The kinetic energy of photoelectrons is measured by determining the flight time of the electrons through a 14 cm long field free drift tube. The timing electronics and the procedures for calibrating spectra were described in a previous paper.I2 Photoelectrons are generated in the ionization of aniline by two photons of 293.8 nm wavelength and by the two-color mechanism using one 293.8 nm and one 200 nm photon. The kinetic energies of the electrons generated by these two ionization mechanisms are very different, so that the spectra are well separated in the time-of-flight measurement. No signal corresponding to ionization with two 200 nm photons is observed. Aniline, purchased from Aldrich and used without further purification, is seeded in 2000 Torr of helium and expanded through a 100 pm nozzle. The mass analysis of the ionized species using our time-of-flight mass spectrometer shows no signature of van der Waals clusters. Results The two-color photoelectron spectrum obtained via the SI origin level at 34 032 cm-',29332-34is shown in Figure 1. The intense origin peak at 7.72 eV and the general appearance of the spectrum up to 8.7 eV agree well with published two-photon ionization photoelectron spectra of the ground electronic state of aniline.9-'4.29Our spectrum shows an additional vibrational progression beginning at about 9.0 eV, which is identified as belonging to the first excited electronic state of *A2 symmetry, by comparison with He I photoelectron s p e ~ t r a . ~ ~The ,~~,~' transition to the 2A2 band is very weak compared to the ground electronic state, its integrated intensity being only 7 f 2% of the transition to the dominant 2 B band. ~ The lowest-energy peak in the 2A2 region is at 9.031 eV, somewhat higher than the adiabatic ionization potential of 8.94 eV found by Debies et al. using He I photoelectron spectro~copy.~'Our value is the average of several measurements, with a standard deviation of 4 meV. Systematic errors are estimated to be less than 10 meV. The 2A2 band exhibits a long vibrational progression with a 65 meV spacing, starting with the 9.031 eV peak. According to the Franck-Condon principle, the vibrational envelope of this progression indicates a difference in the geometry between the intermediate S I state and the ionic 2A2 state. All peaks of this progression except the lowest-energy peak at 9.031 eV are accompanied by a weak peak 39 meV to the high-energy side. All the vibrational peaks observed in this spectral region are listed in Table 1. The two-color photoelectron spectrum via the vibrationally excited Sl(6a') resonance at 34 524 cm-' is reproduced in Figure 2. The general appearance of the band corresponding to the ground electronic state of the cation is again similar to 14332-34
Two-Color Laser Photoelectron Spectroscopy of Aniline+
I\/I
AA A
8
7.5
J. Phys. Chem., Vol. 99, No. 9, 1995 2585
9
9.1
8.5
9.2
9.3
9.4
9.5
9
IONIZATION ENERGY(eV) Figure 1. Photoelectron spectrum of aniline, ionized via the S1(O0o) resonance. The inset shows the spectral region of the first excited electronic state of the cation. The signal-to-noise ratio of the spectrum in the inset is improved by averaging over several measurements.
h
7.5
i
9
8
8.5
9.1
9.2
9
9.3
9.4
9.5
IONIZATION ENERGY(eV) Figure 2. Photoelectron spectrum of aniline ionized via the S1(6a10)resonance.
that of the one-color photoelectron ~ p e c t r u m . ~The - ' ~ additional vibrational progression of the *A2band appears in the two-color spectrum near 9.0 eV. The intensity of the excited ion band is again very weak compared to that of the ground state. All vibrational peaks belonging to the 2A2 state are listed in Table 2. The Franck-Condon envelope of the spectrum via the vibrationally excited resonance is quite different from that via the SIorigin. The peak at 9.031 eV is now the dominant feature, with all higher vibrational peaks being significantly weaker. The stark difference in the vibrational envelopes of the two 2A2 spectra caused by adding one quantum of 6a mode (symmetric
N-ring stretching) to the intermediate electronic resonance indicates that the 6a mode is responsible for the long vibrational progression in the 2A2 spectrum. Apparently there is a significant difference in the equilibrium geometry along the 6a vibrational coordinate between the two potential energy surfaces. A detailed analysis of the 6a progressions as well as the other observed peaks is given below.
Discussion The photoelectron spectra of aniline presented in the previous section demonstrate that excited electronic states of molecular
Kim and Weber
2586 J. Phys. Chem., Vol. 99, No. 9, 1995
and identify the individual peaks of the long progressions to 6a" levels, with the 9.031 eV peak as 6a0, and 6a0 n 65 meV as 6an. The difference between the ionization potential obtained peak intensity position here, 9.031 eV, and the ionization potential of the He I (meVy exptb calcn' assignt photoelectron spectra, 8.94 eV, may be due to hot bands in the 0 100 100 origin (6aolo) amino bend vibration, which would affect only the He I spectra 66 45 24 6a1 of warm molecules in an effusive beam. 1' 100 45 It is often difficult to determine the structural rearrangements 130 32 53 6a2 that are responsible for a progression in a certain vibrational 194 36 44 6a3 254 45 51 6a4 coordinate. Previous studies of electronic transitions of amino 326 32 12 6a5 derivatives indicated that the displacement along the 6a coordinate is due to a shortening of the C-N bond length during LI Peak position relative to the vibrationless ion peak at 9.031 meV. the t r a n ~ i t i o n . A ~ ~shortening of the C-N bond during the Based on a displacement of the equilibrium position of the 6a mode g1'2cm from that in SIstate. in the 2A2 state of 0.6 x ionization is expected from the strong conjugation of the amino nitrogen's lone pair electrons in the i ~ n . ~A. 'normal-mode ~ ions are accessible to two-photon ionization photoelectron analysis of aniline by Song et ~ 1 also. concluded ~ ~ that the 6a spectroscopy. The spectra are amenable to a Franck-Condon vibration has, in comparison with other modes, a fairly large analysis, provided the two laser photons arrive within a time amplitude in the C-N bond stretching motion. These arguments window smaller than the electronic relaxation time of the can be advanced to postulate that it is the shortening of the intermediate state. This condition is met in the work presented C-N bond upon ionization to the 2A2 state that is responsible here, We discuss the Franck-Condon analysis first and then for the extended progressions along the 6a mode. proceed with the discussion of the ionization mechanism All peaks of the 6an progression except for the origin peak responsible for the observed optical transition to the excited with n = 0 exhibit a satellite peak 39 meV to the high-energy ion state. side. These peaks may either be assigned to a short progression Vibrational Assignments and Franck-Condon Envelopes. in a vibrational mode with a frequency of 39 meV built upon The spectrum of the *A2 band via the SIorigin level shows a all 6a modes or to a progression of a vibration with an energy long progression with a spacing of 65 f 1 meV. The spectrum of 104 meV built upon the origin transition. In the first case via the Sl(6a') resonance reveals the same progression, although the satellite peaks may be assigned as combination modes of with a very different intensity distribution. The intensity the type 6a"XI. In this case one expects a peak at 39 meV maximum of the Franck-Condon envelope for the S1(Oo) (6a0X1) with an intensity of 32% of the 2A2 origin (Sa0), spectrum is at the 9.096 eV level of the ion, while the intensity corresponding to the intensity ratio between 6aI and 6a' i39 maximum of the envelope for the Sl(6a') spectrum is at the meV (6a'X'). However, our spectra show no peak above the 9.031 eV vibration. This observation can be explained in a noise floor at 39 meV. Given furthermore the paucity of totally qualitative way by considering the vibrational amplitudes of the symmetric low-frequency modes in aniline ions, we instead oscillations in the intermediate electronic state. The Sl(O0) assign the satellites of the 6an peaks as combination modes of vibration has only the small amplitude associated with the zerothe generic form 6an-'X'. With a vibrational energy for mode point vibration. If the transition to the 2A2 surface is connected X of 104 meV, one expects vibrational peaks at energies of with a change of the equilibrium geometry or the force constants, 6afl-I 104 meV, n = 1,2, ...,as observed in the spectra. The then there will be favorable Franck-Condon factors to excited consistency of the intensity ratios between 6a" peaks and 6a" vibrational states of the 2A2 state. On the other hand, transitions 104 meV peaks within each spectrum supports this assignout of the Sl(6a') level, which has a larger vibrational amplitude, ment. may have a more favorable Franck-Condon overlap with lower Our spectra contain little information about the identity of vibrational levels of the 2A2 band. The fact that the latter mode X. If the ionic 2A2 state were nonplanar in the amino spectrum shows no peak below the 9.031 eV band indicates group, the 104 meV peak could correspond to a transition Io2. that this level is in fact the origin level of the 2A2 state. The frequency of the inversion mode is known to be 52 meV To test this assignment, we compare the observed Franckin 94 meV in S1,29,32-34and 164 meV in the *B1 ~ t a t e . ' ~ . ~ ~ Condon envelopes to stick spectra obtained by modeling the While the frequency of mode X fits with 104 meV well into transitions using harmonic surfaces. Our calculation is based the range of inversion vibrations, one expects a longer progreson the method described by Coon et sion for a transition from the planar S I state to a nonplanar 2A2 which considers both ion state than observed. Alternatively, if the ion in the ?A2 state changes of the vibrational force field and the equilibrium is planar like the SIstate, then the vibration at 104 meV should positions along the vibrational coordinate. The frequency of the 6a mode in the S I state, according to the l i t e r a t ~ r e ~ ~ ,correspond ~ ~ ~ ~ ~ to a totally symmetric mode. Comparison with the and our own resonance ionization spectra, is 61 meV. The vibrational spectra of other electronic states of aniline points to the ring breathing mode 1 as a likely candidate for the short vibrational frequency in the 2A2 ion state is taken to be 65 meV, as obtained from the spectra presented here. The best match progression. The frequency of mode 1 is 110 and 102 meV in of the calculated Franck-Condon factors with the experimental the SO and S I states of neutral aniline,29.34respectively, and the data is obtained by identifying the lowest observed peak at 9.031 frequency in the 2B1 ground state of the aniline cation is 101 meV.9.29 The small variations of the frequency of mode 1 eV with the vibrationless level and with a displacement from between the various electronic states make a good case for the equilibrium position along the 6a coordinate of 0.6 x assigning the 104 meV frequency to the mode 1. The weak g"2 cm. The calculated Franck-Condon factors for the Sl(O0) and S1(6a1)spectra are included in Tables 1 and 2, respectively. peaks at 6a" 39 meV are then assigned to the 6a*-]l1 It is apparent that the calculation reproduces the experimentally progression of a planar ion. This assignment is consistent with the observation that mode 1 is observed to accompany vibraobtained vibrational envelopes of the long progressions of both spectra reasonably well. We therefore assign the lowest tional progressions of mode 6a in electronic transitions of many other aromatic molecule^.^^^^^ observed peak at 9.031 eV to the origin level of the 2A2 band
TABLE 2: Observed Vibrational Peaks in the 2AzState upon Ionization via the S1(6a1)Level
+
+
+
+
Two-Color Laser Photoelectron Spectroscopy of Aniline+
1bl A1 IB2 *Bi 2.42 Figure 3. Electronic configurations of the highest three ~ torbitals . and lowest three antibonding n orbitals in several states: (a) the ground electronic state, (b) the Si state of aniline (exclusive configuration interaction), (c) the ground electronic state, and (d) the first excited electronic state of the aniline cation. The molecular orbital diagram is based on refs 22-25.
The conventional He I photoelectron spectrum of aniline exhibits a vibrational progression with a spacing of 60 meV in the excited ion state.37 Given the relatively large error margins of the peak positions in He I photoelectron spectrum, it is conceivable that this progression is identical to the one we observe at 65 meV. We point out, however, that in the He I photoelectron spectrum the aniline molecule is ionized out of its nonplanar ground state, while in our spectra the ionization proceeds out of the planar SI state. It is therefore possible for the He I spectrum to be dominated by an inversion mode progression that would not be observed in our spectra. The Ionization Mechanism and ConfigurationInteraction Calculations. We now turn to the discussion of the ionization mechanism responsible for the occurrence of the excited ion band. In conventional He I photoelectron spectroscopy, the molecule is ionized out of the ground state of the neutral molecule. In absence of restrictive selection rules comparable to those that govem electronic excitation processes, ejection of electrons out of any occupied orbital is equally possible. While this does not imply that all one-photon ionization cross sections are equal, one often observes all the expected bands in onephoton ionization photoelectron spectra. The situation is more complicated if the molecule is ionized out of one of its excited electronic states using a laser photon of insufficient energy to access low-lying molecular orbitals. We first illustrate the problem by using a simple description of molecular orbitals that excludes any configuration interaction. Figure 3 shows the electronic configuration of the three highest occupied n bonding orbitals, as well as the three lowest unoccupied n orbital^*^-^^ for the neutral molecule in its ground state, the neutral molecule in the electronically excited state, the cation in its ground state, and the electronically excited cation, respectively. The electronic configuration of the neutral molecule ground state is ( lbI)*(la~)~(2b1)*. Ionization to both the ground state cation with a (1b1)*(la2)*(2bl)' configuration and to the first excited cation state with (lb,)*(la2)'(2b1)~is straightforward by an ejection of a single electron from a 2bl or la2 orbital, respectively. The two bands accordingly have similar intensities in the He I photoelectron spectrum. In the absence of configuration interaction, the S1 state is described by a (1bl)2( la2)2(2bl)1(2a2)1 configuration which results from the excitation of one electron from 2bl to 2a2. Ionization to the ground state of the cation is facile by a simple one-electron ejection from the 2a2 orbital. However, in order to generate the ion in its excited electronic state with a ( l b ~ )la2)'(2b1)* ~( configuration, the ejection of an electron from the la2 orbital must be
J. Phys. Chem., Vol. 99, No. 9, 1995 2587
accompanied by a transition of an electron from 2a2 to 2bl. Such one-photon, two-electron processes are known to be very inefficient4I and may be considered forbidden. Weak forbidden bands similar to the excited ion band in our aniline spectra have previously been observed in photoelectron spectra of atoms and diatomic molecules, where they have been explained by configuration interaction in the initial state42 or the final ionic state.43 In order to clarify the contributions of initial state configuration interaction (ISCI) and final ionic state configuration interaction (FISCI) to the appearance of the excited ion band in the aniline spectrum, we calculated the configuration interactions in neutral aniline and its cation using an INDO11 routine with a CAChe interface.43 An optimization of the molecular geometry resulted in vanishing bend angles for the amino inversion mode in the S1 state and both ion states. For the calculations of the configuration interactions we therefore fixed the geometry to be planar. Several trials showed that allowing for nonplanar geometries does not change the results significantly. As expected, the ground state of aniline is found to have a pure (1b,)2(la2)2(2b1)2configuration. The electronic configurations of the cation in its ground state (2B1) and first excited electronic state (2A2) are also predominantly given by (1bl)2(la2)2(2bl)1and (lb1)2(la2)1(2bl)2,respectively, implying negligible configuration interaction in the ion. However, the S I intermediate state of aniline is found to consist 73% of ( lb1)2( 1a2)2(2b1)1(2a2)1and 27% of ( 1b ~ )1~ a2)I ( (2b1)~(3b1)'.While the ionization to the ground state cation proceeds easily out of the dominant contribution with an electron in the 2a2 orbital, it is apparent that excited ions may be generated by ionization out of the orbital with an electron in 3bl and a vacancy in la2. It is thus the initial state configuration interaction, i.e., the mixing of the excited electronic states of the neutral molecule, that is responsible for the appearance of the electronically excited cation. While the INDO/1 calculation correctly predicts a smaller intensity for the excited state cation band than for the ground state band, the calculated relative intensity of the former is 27%, which is higher than the experimentally determined value of 7%. Given the low level character of the INDO11 calculation, we are satisfied with the qualitative agreement between theory and experiment. It is, however, possible that the difference is real, which might point to differences in the cross sections for ionization out of the participating electronic configurations. Summary and Outlook In conclusion, we have demonstrated the applicability of twophoton, two-color ionization photoelectron spectroscopy to excited electronic states of polyatomic molecules. Our spectra show the vibrationally resolved 2Az band of the aniline cation. The ionization potential is 9.031 & 0.010 eV, somewhat higher than the value reported in the literature. An analysis of the Franck-Condon envelope has been performed for the two spectra obtained via different vibrational resonances. We assign a long progression with 65 meV spacings in both spectra to the 6a vibration, implying a geometry change along that coordinate upon ionization from the S1 state. A short progression with a vibrational frequency of 104 meV is tentatively assigned to the mode 1. A simple description of the ionization mechanism neglecting coupling of molecular orbitals reveals the unusual nature of the excited ion band in the photoelectron spectrum via the SI resonance. Similar transitions have previously only been observed in atoms and diatomic molecule^.^'-^^ A comparison with INDO/1 calculations indicates that it is a mixing of the
2588 J. Phys. Chem., Vol. 99, No. 9, 1995
molecular orbitals in the electronically excited neutral molecule that gives rise to the optical transition to the 2A2 state. While there is a qualitative agreement between the theoretical calculation and the observed band intensities, an exact comparison is difficult because of the low level of the theoretical calculation and because ionization cross sections remain unknown. The experiments on aniline presented here have many ramifications. For example, one might wish to increase the resolution of the vibrational spectrum of the 2A2 state by using ZEKE 45 We expect that many other molecules exhibit sufficiently mixed excited electronic states to yield observable spectra. This would allow for excited molecular ion states to be accessible to detailed spectroscopic investigations similar to those currently being conducted on the ground states of molecular ions.I4-I6 The current work also invites the inclusion of higher excited states as intermediate resonances. Since the couplings between higher excited states are generally large, one might expect a multitude of “forbidden” ion bands. Indeed, one might be able to systematically study the mixing patterns of neutral molecular electronic states by mapping them onto the ion states. As nonradiative relaxation processes within highly excited electronic states are generally on a subpicosecond time scale,I9 such a study would require the extension of our picosecond two-photon ionization technology to the femtosecond domain. Current work in our laboratory explores the viability of femtosecond multiphoton ionization photoelectron spectroscopy with far-UV wavelengths. Acknowledgment. We greatly benefited from discussions with Prof. J. Dyke, Southampton, regarding the mechanism of the two-photon ionization. The presented research received partial support from the Petroleum Research Fund. References and Notes (1) Eland, J. H. D. Photoelectron Spectroscopy; Buttenvorths: London, 1984. (2) Endoh, M.; Tsuji, M.; Nishimura, Y. Chem. Phys. Lett. 1984, 109, 35. (3) Tokue, I.; Shimada, H.; Masuda, A,; Ito, Y. J. Chem. Phys. 1990, 93, 4812. (4) Heaven, M.; Miller, T. A.; Bondybey, V. E. J. Chem. Phys. 1982, 76, 3831. ( 5 ) Suh. M. H.: Lee. S. K.: Rehfuss. B. D.: Miller. T. A,: Bondvbev. , , V. E. J. Phys. Chem. 1991, 95, 2727. (6) Lester, M. I.; Zeearski, B. R.; Miller, T. A. J. Phvs. Chem. 1983, 87, 5228. (7) Turner, D. W. Molecular Photoelectron Spectroscopy ; Wiley: London, 1970. (8) (a) Tsuchiya, Y.; Fujii, M.; Ito, M. Chem. Phys. Lett. 1990, 168, 173. (b) Tsuchiya, Y.; Fujii, M.; Ito, M. J. Chem. Phys. 1989, 90, 6965. (9) Meek, J. T.; Sekreta, E.; Wilson, W.; Viswanathan, K. S.; Reilly, J. P.J. Chem. Phys. 1985, 82, 1741.
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