3610
J. Phys. Chem. 1992,96, 3610-3615
Structure and Dynamlcs of Higher Vibronic Levels in the Methyl Radical Rydberg 3s State S. G. Westre, T. E. Gansberg,+P. B. Kelly,* Department of Chemistry, University of California, Davis, California 9561 6
and L. D. Ziegler Department of Chemistry, Northeastern University, Boston, Massachusetts 021 15 (Received: September 13, 1991)
The structure and dynamics of vibronic levels in the methyl radical Rydberg 3s state above the origin were examined by far-ultraviolet resonance Raman spectroscopy. Rotationally resolved Raman excitation profiles were obtained for the CD, [OlOO]level of the Rydberg 3s state. The dissociation rates from the CD, [OlOO] level were observed to be 2 times faster than those from the [OOOO]level. The Raman ezcitation p_rofile analysis yields 47271 cm-’ for the band origin of the [OlOO]-[OlOO]ground to Rydberg 3s absorption (X 2A2/1 B 2A,’). The out-of-plane bend frequency, ui, in the Rydberg 3s state is determined to be 1094 cm-I. The structure and dynamics of the CH, [lOOO]-[0000]vibronic transition were examined. Q branch excitation profiles of the u1 and 2ul Raman features place the [ lOOO]-[OOOO]band origin at 206.85 nm. The Rydberg 3s state symmetric stretching frequency, ul’, is consequently estimated to be 2040 cm-I. The dephasing constant in the [ 10001 level is determined to be approximately 400 cm-I, corresponding to a predissociation lifetime of 13 fs.
-
I. Introduction The methyl radical is the simplest alkyl radical and has been the subject of many experimental and theoretical examinations. However, the structure and dissociation dynamics of the excited electronic states are not well understood. The first spectroscopic study of the methyl radical was performed by Herzberg and Shoosmith.’ Several Rydberg transitions were observed in the far-ultraviolet and vacuum ultraviolet regions. The band origin of the ground to Rydberg 3s absorption (X2A2/1 B 2Al’) was identified in CH, and CD, at 216 and 214.5 nm, respectively. Herzberg suggested two possible assignments for a blue-shifted transition in CD, detected at 211.6 nm. The [OlOO]-[OlOO] vibronic hot band assignment was favored, but the alternative [0200]-[OOOO]band assignment was also possible.2 The later absorption work of Callear and Metcalfe examined the temperature dependence of the 21 1.6-nmband and confirmed the feature as a hot band transition of CD3.’ Callear and Metcalfe tentatively identified-another CD, feature as the [ lOOO]-[0000] transition in the 8-X system. Two CH, bands at 212.7and 207.8 nm that Callear and Metcalfe were not assigned. were observed The CH, 8-X absorption band origin consists of two diffuse maxima while the corresponding band of CD, shows broad, but clearly resolved, rotational structure.Ip2 The diffuseness in the spectra was attributed to predissociation of the B state by H (or D) atom tunneling.2 The CD3 [OlOO]-[OlOO]absorption band was observed to be more diffuse than the origin band, suggesting the degree of predissociation increases with the amount of vibrational energye2 An ab initio calculation on the methyl radical was performed for the ground-state and 3s, 3p,,, Rydberg states by Yu et alO4 Investigation of the H2C-H fragmentation pathways for the Rydberg 3s state of CH, showed-the methyl radical B state predissociation is analogous to the A state (Rydberg 3s) predissociation of ammonia. The out-of-plane bending potentials in the ground and B electronic states are perturbed by a pseudo-Jahn-Teller interaction. The interaction was first observed by Yamada et a1.,5 who identified the 1-0, 2-1, and 3-2 transitions in u2 using diode laser spectroscopy. The CH, u2 potential was found to contain a large quartic contribution, causing the methyl radical to have an anomalously low bending frequency and the v2 manifold to exhibit
-
* Author to whom correspondence should be addressed. ‘Current address: Department of Chemistry, Stanford University, Stanford, CA 94305.
negative anharmonicity. Yamada et al. accounted for the low bending frequency and negative anharmonicity in the CH, u2 manifold by a pseudo-Jahn-Teller vibronic coupling of the 2 and B electronic states through the u2 vibration. Frye et aL6and Sears et ala7examined the CD, u2 bending potential, identifying the 1-0, 2-1, 3-2, and 4-3 vibrations. The predissociation dynamics of the 8 state [OOOO]-[0000] levels for CH, and CD3 were directly examined using resonance Raman spectroscopy.8 Raman excitation profiles in resonance with the Rydberg 3s [oooO]level were obtained for CH, and CD,. The Raman excitation profiles yielded rotationally dependent subpicosecond lifetimes for the methyl radical. The lifetimes and corresponding tunneling rates were simulated with a one-dimensional cubic potential barrier model to extract the height, width, curvature, and position of the barrier to dissociation. In the current work we present a study of the vibrational structure and dynamics of higher vibronic levels in the methyl radical Rydberg 3s state. The predissociation rates and excited-state vibrational frequencies determined in this study further elucidate the nature of the methyl radical Rydberg 3s state. 11. Theory
The intensities of resonance Raman rovibronic features yield information regarding the structure and dynamics of the resonant excited state. The intensities of resonance Raman transitions can be described by the Kramers-Heisenberg-Dirac (KHD) sum over all states f o r m a l i ~ m . ~ The J ~ intensity of a Raman transition is given in the KHD formalism by the Raman scattering cross section:
(1) Herzberg, G.; Shoosmith, S. Cun. J . Phys. 1956. 34, 523. (2) Herzberg, G. Proc. R. SOC.London 1961, 262A, 291. (3) Callear, A. B.; Metcalfe, M. P. Chem. Phys. 1976, 14, 275. (4) Yu, H. T.; Sevin, A.; Kassab, E.;Eveleth, E.M . J . Chem. Phys. 1984, 80,2049. ( 5 ) Yamada, C.; Hirota, E.; Kawaguchi, K. J . Chem. Phys. 1981, 75,5256. (6) Frye, J. M.; Sears, T. J.; Leitner, D. J . Chem. Phys. 1988.88, 5300. (7) Sears, T. J.; Frye, J.; Spirko, V.; Kraemer, W. P. J. Chem. Phys. 1989, 90, 2125. (8) Westre, S. G.; Kelly, P. B.; Zhang, Y. P.; Zielger, L. D. J . Chem. Phys. 1991, 94, 270. (9) Ziegler, L. D.; Chung, Y. C.; Wang, P.; Zhang, Y. P. In Time Re-
solued Spectroscopy; Clark, R. J. H., Hester, R. E., Eds.; Wiley: New York, 1989 and references cited therein. (IO) Ziegler, L. D. J . Chem. Phys. 1986, 84, 6013.
0022-3654/92/2096-36 l0$03.00/0 0 1992 American Chemical Society
The Journal of Physical Chemistry, Vol. 96, No. 9, 1992 3611
Methyl Radical Rydberg 3s State where a is the fine structure constant, N is the number of scatterers per unit volume, us is the scattered frequency, Pg is the population of the initial level, and corresponds to the isotropic ( k = 0), antisymmetric ( k = l ) , and anisotropic ( k = 2) polarizability tensor elements. The unprimed, primed, and double-primed superscripts refer to the initial, resonant, and final states. For a rovibrational Raman transition, ( u J K ) (u”J”K”), dominated by a single parallel polarized resonant vibronic transition (gv e/), the contribution to the Raman intensity from the polarizability tensor elements, IC$’, is given by8-l0
-
rc$2 =
(U
+
1x21”
+ l)IC(-l)JiU+ l ) [kJ , J” J
-
]
J’
Equation 2 shows the Raman scattering intensity is a function of the vibronic transition moments, (Mo)$ and (Mo)s,, the energy difference between the resonant transition and the excitation laser ~ j K j Kt yo), and the dephasing constant associated with the resonant molecular transition, r. The excitation wavelength dependence of the intensity of Raman features is a function of both the structure and dynamics of the resonant state. The nature of the resonant rovibronic state is experimentally examined by mapping the changes in intensity of the features in the Raman spectrum as a function of excitation wavelength. In the case of resonance with a single vibronic state, the Raman excitation profile (REP) may be fit to eq 2 using only (up,e,, uJKJr - yo) and r as adjustable parameters. Energy level spacing information is obtained from the value of the rovibronic band origin (up,e,, ~ j K j K t ) determined , by the REP analysis. At our experimental pressures, electronic dephasing in the methyl radical Rydberg 3s state is dominated by TI pmcesses, Le., the population decay due to the fast tunneling predissociation. Thus the dephasing constant reflects the molecular dynamics of the resonant rovibronic level in Rydberg 3s state. The rovibronic specific dephasing constant is related to the excited-state lifetime by 7 (27rcI”)-l. The 3-j and 6-j symbols in eq 2 describe the conservation of angular momentum and are a formal expression of the rotational resonance Raman selection rules.”’ The triangular condition on the upper arguments of the 3-j symbol controls the rotational Raman scattering so that S branch Raman lines only occur through resonant rotational levels where J’ = J + l.9J0 The triangular condition on the upper argument of the 6-j symbol allows only the anisotropic ( k = 2) polarizability tensor elements to contribute to the S branch rotational Raman scattering process. Equation 2 is simplified by K = K’ = K” due to the hK = 0 rotational selection rule for parallel electronic transitions. The lifetimes reported here are necessarily averaged over all populated values of the K rotational quantum number due to the instrumental resolution of our apparatus.
+
+
+
111. Experimental Section
The far-ultraviolet resonance Raman system used in the examination of the higher vibronic levels of the methyl radical Rydberg 3s state is similar to that used in the investigation of the predissociation dynamics of the methyl radical Rydberg 3s [oooO] level.8 Tunable far-ultraviolet light is obtained by harmonic generation techniques from a 20-Hz Nd:YAG-pumped dye laser system. The dye laser was operated with DCM or a DCM/Kiton Red mixture to produce laser output from 61 2 to 650 nm. The dye laser fundamental was doubled in KD*P by type I second harmonic generation. The @axis of the KD*P crystal was rotated 20” from vertical to provide a component of the residual dye laser fundamental polarized parallel to the resultant second harmonic. Tunable third harmonic radiation was produced by type I phase matching of the second harmonic and fundamental in 8-barium borate. The third harmonic emerged with its electric vector 20” ( 1 1) Zare, R. N. Angular Momentum; Wiley: New York, 1988.
from vertical and was subsequently polarized vertically by a quartz stacked plate polarizer. The probe beam was focused into the sample region by a 15-cm focal length lens. The methyl radicals were generated by 266-nm photolysis of methyl iodide. The photolysis beam is produced by the Nd:YAG laser with its electric vector oriented at right angles to the probe beam. The intensities of both the photolysis laser and the probe laser were monitored by photodiodes. The laser powers varied by less than 10%. A carrier gas (methane or argon) was bubbled through liquid methyl iodide to produce a methyl iodide/carrier gas mixture at 22 O C . The resultant gas mixture was introduced into the sample region through a modified Bosch fuel injector operated at a total pressure of 820 Torr (approximately 60 Torr above atmospheric pressure). The equilibrium vapor pressure of methyl iodide at 22 “C is approximately 350 Torr, and so the gas mixture consisted of 350 Torr methyl iodide and 470 Torr of the carrier gas. The fuel injector sample delivery system was employed in an effort to conserve deuterated methyl iodide. The sample flow was mutually orthogonal to the propagation directions of the probe and photolysis lasers. The probe laser arrived in the sample region 15 ns after the arrival of the photolysis beam to allow collisional cooling of the nascent methyl radical population distribution. The Raman scattered light was collected in backscatter geometry by a 5-cm focal length, f/2.0 spherical mirror. The collected Raman light was focused onto the slits of 1.0-m monochromator, passed through a quartz polarization scrambler, and dispersed in third order by an 1800 groove/mm grating. The light was detected using either a Hammamatsu R166UH solar blind photomultiplier tube or a Princeton Instruments ICCD detector. Frequency calibration was accomplished by use of atomic lines from a low-pressure mercury/argon lamp. The Raman signal, calibration lamp signal, and laser intensity signals were recorded using an A/D converter and stored on an IBM PC/AT computer.
IV. Results and Discussion IdenWication of the-Methyl Radical [OlOOHOlOO]Band. The first study of the CD, B-X absorption spectrum observed a band at 211.6 nm which could be assigned as either the [OlOO]-[OlOO] or [O2OO]-[OOOO] transition.’$* Callear and Metcalfe, examined the temperature dependence of the 21 1.6-nm band and confirmed that the feature is a hot band transition of CD,, corroborating the [OlOO]-[OlOO] assignment. However there was no direct spectroscopic evidence of the identity of this band. Resonance Raman spectroscopy can determine the identity of vibrational features in an absorption spectrum because the resonance enhancement is sensitive to the nature of the excited-state level. The B (Rydberg 3s) electronic state has 2Al’ symmetry while the ground state is of *A2” symmetry. The fundamental of the u2 vibration has a; symmetry in the D3,, point group. Group theoretical considerations dictate that both the [0200]-[0000] and [OlOO]-[OlOO] vibronic absorptions are only allowed as z polarized transitions since the z dipole moment operator has a; symmetry in the D3*representation. The polarizability tensor elements, eq 2, incorporate the vibronic dipole transition moments from the initial to intermediate states, (Mo)$ and intermediate to final states, (Mo)$,. Therefore the Raman scattering is constrained by group theory to transitions that mcur between vibronic levels of the same symmetry. Raman spectra in resonance with the [0200]-[0000] vibronic transition would exhibit intensity in the even out-of-plane bending overtones 2uz, 4u2, and possibly 6u2. Combination modes of the al’ symmetric stretch, u I , and the even overtones of u2 would be expected to appear as [1200]-[0000], [ 14001-[0000], [2200]-[OOOO], etc. In contrast, resonance with the [OlOO]-[OlOO] absorption would yield intensity in the resonance Raman spectrum in the odd-numbered overtones of the out-of-plane bend such as [0300]-[0100] and [0500]-[0100]. The combination modes of the symmetric stretch and out-of-plane bend appearing in the Raman spectrum would be [1100]-[OlOO], [ 13001-[0 1001, [2 1001-[0 1001, etc. The Raman spectrum of CD, in resonance with the vibronic feature at 21 1.54 nm is shown in Figure 1. Several features
3612 The Journal of Physical Chemistry, Vol. 96, No. 9, 1992
Westre et al. 1 .oo
.-z e
.-t
g
-d
0.80
C
.-t 0.40
0.00
I
I
-I
i
"2
l
cw
I
li
/
0.60
0.40
1
K
0.20
0.20
0.00
0.00
,
1
0.0
2000.0 Wavenumbers
4000.0
Figure 1. Resonance Raman spectrum of CD3 with 21 1.54-nm excita-
tion.
TABLE I: Observed Methyl Radical Stretching Frequencies in the Raman Spectrum Excited in Resonance with the [OlOOHOlOO] Vibronic Band
vibrational transition [1100]-[OlOO] [2 100]-[0100] [0300]-[ 0 loo] [0500]-[0100] [ 13001-[Ol OO] [1500]-[OlOO]
CD3 freq/cm-' 2156.4 4293.1 1054.0 2212.6 3204.7 4360.7
0.0
-4
2000.0 4000.0 Wavenumbers
6000.0
Figure 2. Resonance Raman spectrum of CH3 with 212.99-11111 excitation. I
1.00
-
CHI freq/cm-'
(20) (20) (18) (10) (20) (20)
2993.9 (10) 5949.5 (27) 1410.7 (10) 4382.2 (31)
"The number in parentheses represents the standard deviation of the measurement. belonging to the methyl radical are observed in the Stokes portion of the spectrum. No anti8tokes Raman lines were observed. In particular, Stokes shifted Raman lines are found at 1054.0 (18) and 2212.6 (10) cm-I. Comparison to the CD3infrared diode laser study of the v2 potential by Sears et al.' shows that the 1054- and 2212-cm-l Raman features correspond to the Raman [0300][OlOO] and [0500]-[OlOO] hot band transitions. The presence of the [0300]-[OlOO] and [0500]-[0100] overtones in the resonance Raman spectrum provides direct evidence that the absorption feature centered at ca. 211.6 nm is due to the [OlOO]-[OlOO] vibronic transition. No other assignment of the absorption spectrum satisfactorily explains the presence of the [0300]-[OlOO] and [0500]-[OlOO] vibrations in the Raman spectrum. Table I summarizes the frequencies and assignments of the Raman features. As expected, combination modes of the symmetric stretch and odd overtones of v2 are also observed. The [1300]-[OlOO] and [1500]-[0100] combination vibrations are a t 3204.7 (20) and 4360.7 (20) cm-I. The band at 2055 cm-' is probably a combination band of v4 and v2such as [0102]-[0100]. The previous Raman studyI2 in resonance with the CD, [OOOO]-[0000] absorption identified the 2v4 overtone at 2055.5 (18) cm-I. An unidentified CH3 absorption centered a t 212.7 nm was observed by Callear and Metcalfe., Figure 2 is the Raman spectrum of the protonated methyl radical at 212.99 nm. The bands at 530 and 1250 cm-I are readily identified as the v3 and u2 vibrations of methyl iodide. The peak a t 2917 cm-' is the symmetric stretch, vlrof methane. The methyl radical feature found at 1410.7 (9) cm-I is in good agreement with the [0300]-[OlOO] vibrational spacing, 1412.7 cm-', determined by the diode laser work of Yamada et alSs The remaining methyl radical features at 2994.0 (IO), 4381.2 (21), and 5949.5 (33) cm-I are identified as the [1100]-[OlOO], [1300]-[OlOO], and [2100]-[OlOO] combination bands. The vibrational frequencies of CH3 in the Raman spectrum when excitation is in resonance with the 212.7-nm absorption feature are summarized in Table I. The appearance of both 3v2 and the nvI v2 combination modes originating with 1 quanta of excitation in v2 indicate that the CH3
+
(12) Westre,
S.G.;Kelly, P.B. J . Chem. Phys. 1989, 90, 6977.
2200.0 2300.0 Wavenumbers
2400.0
Figure 3. Resonance Raman spectrum of the CD, Y, rovibronic region. Excitation resonant with the R branch of the absorption results in S
rovibrational lines that are intense and well resolved. radical absorption feature a t ca. 213 nm is due to the [01001-[0 1001 hot band vibronic transition. R o t a t i d y Resolved Dynamics of the CD,[OlOO] Level of the Rydberg 3s State. The dynamics of the Rydberg 3s state [OlOO] level of CD3 were examined by analyzing the rotationally resolved Raman excitation profiles resonant with the R branch of the [OlOO]-[OIOO] absorption. Figure 3 shows that the S branch rovibrational Raman features are intense and well resolved. Excitation profiles were obtained by measuring the intensities of the J resolved S branch features with excitation at 20 wavelengths evenly spaced between 210.989 and 21 1.638 nm. The S branch excitation profiles were modeled according to the Raman intensity expression in eqs 1 and 2 to extract the J' specific dephasing constants. Theoretical modeling of the CD, Raman excitation profiles requires knowledge of the population of the rotational levels. Equations 1 and 2 show that the intensity of a rotational Raman feature is a linear function of the population of the initial rotational level. A Boltzmann distribution at T = 300 K is used to calculate rotational populations. The 266-nm photolysis of methyl iodide produces rotationally and vibrationally hot methyl radicals. The delay between the photolysis and probe beams allows rotational and vibrational cooling of the methyl radicals and randomization of the methyl radical orientation by collisions with the buffer gas. The experimental temperatures and pressures used allow approximately 90 CH3 (or CD,) collisions with the methane carrier gas and 80 collisions with the remaining methyl iodide precursor in the 15 11s between photolysis and the arrival of the probe laser.8 The methyl iodide is photolyzed in an effusive gas flow and so the minimum temperature the radicals can attain is the ambient temperature of the laboratory. Figure 4 shows CD3 rotational spectra obtained after 15- and 40-11s delays of the probe pulse following the photolysis pulse. The lack of features in the difference spectrum demonstrates there is no discernible change in the relative intensities of the rotational features at the two delay
The Journal of Physical Chemistry, Vol. 96, No. 9, 1992 3613
Methyl Radical Rydberg 3s State
0
3
Difference
2200.0
2250.0
2300.0
__e
2350.0
II-
< u w W > I m -
3
W
a
Wavenumberr
Figure 4. Resonance Raman spectra of CD3with 15- and 40-11s delays betwecn the photolysis and probe beams. The spectrum with 40-ns delay is normalized to the 15-ns delay spectrum by a 2.64 scaling factor. The difference spectrum contains no methyl radical rotational features, indicating the methyl radical population is at room temperature after the 15-11s delay.
TABLE II: J’Specif~cDephasing Constants, r(fwhm), and Lifetimes for the CD, [OlOOl Level of the Rydberg 3s State resonant resonant rotational rotational level, J’ r/cm-l lifetime/fs level, J’ r/cm-l lifetime/fs 350 7 26.0 3 15.0 200 250 8 28.0 4 21.0 190 5 19.5 270 9 27.0 200 6 24.0 220 10 35.0 150
times. The difference of the CD,spectra shown in Figure 4 reveals that the rotational temperature is identical in the 15- and 40-11s delayed spectra. Thus the methyl radicals are rotationally cooled to room temperature by 15 ns. The excitation profile of an S(J) Raman feature reflects the dynamics of the J’ = J 1 resonant rotational level. The S(J) Raman lines are enhanced only by resonance with the R(J) absorption features (J’ = J 1). Measurements of the S(J) Raman intensities determine the spectral characteristics of the R(J) absorption features and thus the energy and dephasing associated with the J’ = J 1 rovibronic level. The appearance of the [0300]-[OlOO]Raman transition a t 1054 cm-I and the lack of the [0200]-[0000]Raman transition at 956 cm-l demonstrate that CD3 spectra produced with 21 1-nm excitation derives enhancement solely from resonance with the [OlOO]Rydberg 3s B *A1’vibronic level. The Raman excitation profiles for S(3) and S(7) are shown in Figure 5 . The intensities are relative to the Raman signal from v1 of the methane buffer gas which is used as an internal intensity standard. The CD, lifetimes exhibit a moderate J’dependence with the lifetime decreasing from 350 fs in the J’ = 3 level to 150 fs for the J’= 10 level. Table I1 is a summary of the dephasing constants and the associated excited-state lifetimes for the J’ = 3 through J’ = 10 rotational levels in the Rydberg 3s state. The rotational dependence of the dissociation process is attributable to a reduction of the effective barrier height by centrifugal forces in the rotating m o l e c ~ l e .The ~ ~ ~excited-state lifetimes of the CD3 [OOOO] rovibronic levels of the Rydberg 3s state similarly reflect rotational enhancement of the dissociation.8 The CD, (01001 vibronic level dissociation rates (=1/7)are more than twice as fast as the dissociation rates from the CD, [OOOO] origin level.* In contrast, the dissociation rates from t h e [OlOO]level of the ND3 Rydberg 3s state were measured to be 3 times slower than those from the [OOOO] level.I3J4 Due to the
RAMAN E X C I T A T I O N WAVELENGTH NM
-
Figure 5. CD3 Raman excitation profiles and modeling fo? S(3) (*) and S(7) (0) in resonance with the [OlOO]-[OlOO] X Z A T B 2Al‘ vibronic absorption. The rotationally specific I”s are 21 and 28 cm-’ for J’ = 4 and 8, respectively. b
a
+ +
+
(13) Douglas, A. E. Discuss. Furuduy SOC.1963, 35, 158. (14) Ashfold, M. N. R.;Bennett, C. L.; Dixon, R. N. Chem. Phys. 1985, 93, 293. (15) Heller, E. J. Acc. Chem. Res. 1981, 14, 368. (16) Heller, E. J.; Sundberg, R.L.; Tannor, D. J. Phys. Chem. 1982,86, IILL.
REA”
COORDINAlF
R U C I X ) N COORDlNAlE
Figure 6. Potential energy surfaces for the ground and Rydberg 3s states of ammonia (a) and the methyl radical (b) showing the correlation of the dissociation pathways and product states. The broken lines in (a) correspond to nonzero out-of-plane angles, 8, while the solid lines are the ammonia potential surfaces along the 0 = 0 coordinate. The methyl radical potential energy surfaces are not strongly affected by the 8 coordinate, and so the solid lines in (b) represent the potential energy surfaces for both the 8 = 0 and 8 # 0 coordinates.
complexity of ab initio calculations for open-shell systems, there is not a good theoretical potential energy surface available for the methyl radical. However, much can be learned by considering the similarities and differences between the methyl radical and ammonia Rydberg 3s states. Experimental studies and ab initio calculations4~s~9~~7-22 have shown that several aspects of the Rydberg 3s states of ammonia and the methyl radical are similar. The equilibrium geometries (17) Vaida, V.; McCarthy, M. I.; Engelking, P. C.; Rosmus,P.; Werner, H. J.; Botschwina, P. J. Chem. Phys. 1987, 86, 6669. (18) Muller, J.; Canuto, S. Chem. Phys. Lett. 1980, 70, 236. (19) Rosmus, P.; Botschwina, P.; Werner, H.-J.; Vaida, V.; Engelking, P. C.; McCarthy, M. I. J. Chem. Phys. 1987, 86, 6677. (20) McCarthy, M. I.; Rosmus, P.; Werner, H. J.; Botschwina, P.; Vaida, V. J. Chem. Phys. 1987, 86, 6693. (21) Biesner, J.; Schnieder, L.; Schmeer, G.; Ahlers, G.; Xie, X.; Welge, K. H.; Ashfold, M. N. R.; Dixon, R. N. J. Chem. Phys. 1988, 88, 3607. (22) Biesner, J.; Schnieder, L.; Ahlers, G.; Xie, X.; Welge, K. H.; Ashfold, M. N. R.;Dixon, R. N. J. Chem. Phys. 1989, 91, 2901 and references cited therein. (23) Parker, D. H.; Wang, Z. W.; Jansen, M. H. M.; Chandler, D. W. J. Chem. Phys. 1989, 90, 60.
3614 The Journal of Physical Chemistry, Vol. 96, No. 9, 1992 of both molecules are planar (D,,,) in the 3s Rydberg state, and the [OOOO] and [OlOO] vibronic levels are predissociated by the tunneling of a hydrogen nucleus through a potential barrier on the excited-state ~ u r f a c e . ~In, the ~ ~ previous one-dimensional cubic barrier analysis,*it was assumed that the [oooO]level of the methyl radical and ammonia Rydberg 3s states dissociate along planar (C2J geometries. The methyl radical was found to have a 2200-cm-I barrier at Rc-D = 1.38 A whereas the ammonia dissociation barrier is 2300 cm-I at R p D = 1.40 A. However there is a significant difference between the Rydberg 3s states in relation to the correlation of the dissociation pathways with product states. The ammonia Rydberg 3s surface was theoretically examined along the bond stretch and angle bend coordinate~.l~-~~ Ammonia molecules dissociating on the Rydberg 3s potential energy surface with placar (C,”) geometries were found to correlate with H(2S) NH,(X ,B1) (or D ND,) while nonplanar geometries (C,) correlateyith H (or D)(,S) and the first excited state of NH, (or ND2)(A 2Al). The dissociation of the ammonia X ‘A, ground state in the strictly planar geometry correlates t,o the H (or D)(,S) and the first excited state of NH, (or ND,)(A ,A,). Thus in C,, geometry there is a crossing of the Rydberg and ground-state curves along the dissociation coordinate.4~17-22~24 The correlation of the ammonia dissociation pathways with product states is shown in Figure 6a. The change in correlation as a function of dissociation geometry results in a conical intersection of the ground and Rydberg 3s states of ammonia. The conical intersection and change in correlation can cause the barrier for the symmetric C2, dissociation to be lower than the barrier for the C, d i s s ~ c i a t i o n . ~ ~ - ~ ~ ~ ~ ~ The ground and Rydberg 3s states of the _methyl radical have also been studied by ab initio method^.^ The X state of the methyl Cadical was found to correlate with the 3Blstate of CD2 while the B state correlates with ’Al CDI. Both the C, and C , dissociation paths of the Rydberg 3s state correlate to the same IAl excited state of CD,. The product-state correlations of the methyl radical dissociation pathways _areillustrated in Figure 6b. The correlation of the methyl radical B state does not change with geometry, and so there is no conical intersection of the X and B states. The invariance of correlation for the methyl radical can cause differences between the methyl radical and ammonia Rydberg 3s surfaces. The barrier to dissociation on the methyl radical Rydberg 3s surface is not expected to be as different along the C, and C, pathways as for ammonia and thus not as strongly dependent on the out-of-plane angle coordinate, 0. The relative dissociation rates for CD3 and ND3 reflect the different t9 dependencies of the methyl radical and ammonia barriers with respect to the dissociation pathways. In the planar Rydberg 3s state, ammonia molecules in the [OOOO] origin level tend to dissociate with planar geometries to form ground-state ND,. The [OOOO] excited-state wave function for u2 has a maximum amplitude and thus a maximum probability on the fJ = 0 coordinate, and the tunneling will occur where the barrier is lowered and narrowed25due to the conical intersection of the excited-state and ground-state surfaces. The [OlOO] level wave function has a node at fJ = 0 and an amplitude at 0 # 0 from which tunneling along the N-D dissociation coordinate corresponds to the C,dissociation pathway. The [OlOO] level of ND, dissociates more slowly than the [0000] level, which indicates that the C, dissociation pathway along fJ # 0 proceeds through a barrier higher than the C, pathway along 0 = 0. In CD, both dissociation pathways correlate to the same products and the barrier for the C, pathway is not expected to be significantly greater than the barrier for the C,, pathway. Thus the increased energy of the [OlOO] level causes a faster dissociation than the [OOOO] level in CD3. Vibrational Structure of the Rydberg 3s [OlOO] Level of CDp The intensity expressions for Raman scattering, eqs 1 and 2, show that modeling of the Raman excitation profiles yields an estimate
+
+
(24) Woodridge, E. L.;Ashfold, M. N. R.;Leone, S. R.J . Chem. Phys. 1991, 94, 4195.
(25) Dixon, R.N. Mol. Phys. 1989, 68, 263.
Westre et al.
0 0.40 ’50
I
0.00 200.00
205.00
2 10.00
2 15.00
Excitation Wavelength /nm
Figure 7. Raman excitation profile of the CHI radical 2ul Q branch enhanced by resonance with the [lOOO]-[OOOO] vibronic absorption. The 2u, intensity is normalized to u1 of CH4. Modeling (solid line) yields a vibronic band origin at 206.85 nm with a 400-cm-’ dephasing constant.
of the vibronic band origin of the resonant electronic transition. The analysis of the CD, [OlOO] Raman excitation profiles results in a [OlOO]-[OlOO] vibronic band origin of 47271 cm-’. The frequency of the out-of-plane bending mode in the Rydberg 3s state may be calculated from the band origin determined by our Raman excitation profile analysis of the CD3 [oooO]-[oooO] banda and the known v2 ground-state frequency, 457.8 cm-Is6 The u2 frequency is thus determined to be 1094 cm-l in the Rydberg 3s state. The u2 frequency estimated from the absorption spectrum by Callear and Metcalfe, is 1090 cm-I. The CD3 Rydberg 3s v2 frequency is comparable to the out-of-plane bending frequency in the Rydberg 3p, electronic state, 1036 cm-I, determined by the (2+1) REMPI study of Parker et aL2, The diode laser study of the CH, u2 manifold revealed that the v2 manifold in the ground electronic state contains a large quartic contrib~tion.~ The large quartic force constant results in negative anharmonicity and a low bending frequency in the u2 potential. Yamada et alespostulated a pseudo-Jahn-Teller vibronic interaction between the ground and Rydberg 3s states to account for the quartic nature of the u2 potential. Our experimental value for v i is comparable to that predicted on the basis of Yamada’s Jahn-Teller analysis. Observation of higher levels is necessary to examine the full effects of the vibronic interaction on the Rydberg 3s state. The Structure and Dynamics of the CH3 [lO?O]-Rydherg 3s State. Callear and Metcaife’s study of the CH3 B-X absorption system revealed a feature at approximately 207.9 nm. The resonance Raman spectrum of CH3 at 207 nm contains only the symmetric stretching bands uI and 2uI. The fundamental and first overtones of the symmetric stretch are roughly equal in intensity. In comparison, the u1 vibration exhibits 7 times the intensity of the 2ul feature-fo? Raman spectra resonant with the CH3 [OOOO]-[0000] B-X absorption. The intensity enhancement of 2ul relative to v 1 at 207 nm is evidence of resonance with one quanta of the symmetric stretch in the Rydberg 3s state. The observation of u1 and 2ul with equal intensities and the lack of anti-Stokes vibrations indicate the 207-nm_fe_ature is due to the [ lOOO]-[0000] vibronic transition in the B-X system. Raman excitation profiles of the u1 and 2ul Q branch features in resonance with the [lOOO]-[0000] absorption were obtained in order to examine the dynamics of the [ lOOO] Rydberg 3s level. The 2ul excitation profile is shown in Figure 7. The v 1 and 2ul excitation profiles are similar in shape although the u1 excitation profile is a factor of 1.7 more intense. The band origin of the [lOOO]-[oooOJ absorption is determined to be 206.85 nm from the modeling of the uI and 2ul REPS. The symmetric stretch in the Rydberg 3s state, ul’, is determined from the [lOOO]-[0000] and [OOOOJ[0000] band origins to be 2040 cm-I. The dephasing constant derived from the [ 10001 level Raman excitation profile fitting is estimated to be 400 cm-l fwhm, corresponding to an excited-state lifetime of approximately 13 fs. The dissociation lifetime from the [ 10001 level of CH, is on the order of one vibrational period, 16 fs, of the vl’ stretching mode.
J. Phys. Chem. 1992, 96, 3615-3621
V. Conclusions The structure and predissociation dynamics of vibronic levels in the methyl radical Rydberg 3s state above the origin level were examined using Raman excitation profile techniques. The [OlOO]-[OlOO] and [ lOOO]-[OOOO] absorption features have been conclusively identified by the vibrational features observed in the reSOnance Raman spectra. The band origin for the [OlOO]-[OlOO] absorption is determined to be 47271 cm-I from the modeling of the rotational excitation profiles. The frequency of the out-of-plane bend in the Rydberg 3s state is estimated to be 1094 cm-’. The analysis of the v 1 and 2vl Q branch excitation profiles in resonance with the [ lOOO]-[oooO] level of CH3 determines the band origin to be 206.85 nm, resulting in a symmetric stretching frequency, vl’, of 2040 cm-l in the Rydberg 3s state. The J-resolved experimental CD3 [OlOO] dissociation rates are consistent with a predissociation barrier in the methyl radical Rydberg 3s state that is less dependent on the degree of out-ofplane vibrational excitation than that in ammonia. One significant difference between the methyl radical and ammonia Rydberg 3s state dissociations is the invariance of product-state correlation with methyl radical dissociation geometry. The barrier to dissociation in the ammonia Rydberg 3s state appears to increase as a function of the o;t-of-plane angle due to the_conica_lintersection of the X and A states. In contrast, the X and B states of the methyl radical do not intersect and so the methyl radical barrier is not a strong function of the degree of out-of-plane excitation. The methyl radical dissociation bamer for the Rydberg 3s state is experimentally found to be less dependent upon the 8 coordinate than that of the corresponding state in ammonia. The extremely fast methyl radical dissociation dynamics of the [ lOOO] level indicate the level is quasibound in agreement with our previous analysis of the barrier to dissociation on the Rydberg 3s surface. Our previous analysis of the predissociative methyl radical and ammonia Rydberg 3s states* explicitly ignored all 8 dependence of the barrier function and any coupling of the out-of-plane bend
3615
to the dissociation coordinate. The one-dimensional model was useful for examining the dissociation of the vibrationless [OOOO] level. However, the current experimental dissociation rates for CD, and the large body of work on the ammonial3-I9 ;Y vibronic levels clearly demonstrate the necessity of including the coupling of the out-of-plane bending coordinate to the dissociation. Approximate models that restrict the internal molecular degrees of freedom are extremely useful in analyzing limited experimental data sets. However, ab initio calculations that provide insight into which vibrations and rotations are important in the dissociation would improve our understanding of molecular excitg states. The available ab initio calculation on the methyl radical B state surface examined the photochemistry only along the dissociation coordinate. In addition to the ammonia work previously described, there are few examples (methyl nitrite26 and ketenez7) of the combination of multidimensional a b initio calculations and experimental vibrational and rotational dissociation information to produce an accurate picture of a molecular photoactive surface. It is our hope that the new experimental methyl radical Rydberg 3s dissociation information presented here will encourage further theoretical examinations of the interesting photochemistry of the excited states of the methyl radical. Acknowledgment. Support for the work from the NSF (Grant CHE-8923058), the NIEHS (Grant lP42-ES04699), and the Universitywide Energy Research Group of the University of California (Grant 444024) is gratefully acknowledged by P.B.K. L.D.Z. acknowledges the support of the N S F (Grant CHE8918418) and the donors the Petroleum Research Fund, administered by the American Chemical Society. Registry No. Methyl radical, 2229-07-4;Methyl-d, radical, 212244-3. (26)Suter, H.U.;Bruhlmann, U.; Huber, J. R. Chem. Phys. Lett. 1990, 171. 63 and references cited therein. (27)Alen, W. D.; Schaefer, H. F., 111; J . Chem. Phys. 1988,89,329;1987, 87,7076;1986 84,2212.
Solvatochromlsm of a Typical Merocyanine: Stilbazoiium Betaine and Its 2,6-DI-tert-butyi Derivatlve J. Catalln,*,+E. Mens,+ W. Meutermans,t and J. Elguerof Departamento de Quimica Flsica Aplicada, Facultad de Ciencias, Universidad Autdnoma de Madrid, Canto Blanco, 28049 Madrid, Spain, and Instituto de Quimica MZdica, CSIC, Juan de la Cierva, 3, 28006 Madrid, Spain (Received: September 23, 1991; In Final Form: January 2, 1992)
The absorption and emission spectra of DTBSB (the 2,6-di-tert-butyl derivative of stilbazolium betaine (SB)), in its neutral and protonated forms, have been measured in 28 solvents. Those corresponding to the parent SB (4’-hydroxy-l-methylstilbazolium betaine or 4- [ 2-( 1methyl- 1,4-dihydropyridinylidene)ethylidene]cyclohexa-2,5-dien-1-one)) have been recorded in the solvents necessary for comparison. The effect of the ortho bulky groups is to efface the effect of the solvent acidity. The remaining polarity effect on the solvatochromismis very weak but is nevertheless always negative, even in apolar solvents. In consequence, the behavior of SB is not in contradiction with Onsager’s models of general solvent effects. Stilbazolium betaines cannot be used as probes of solvent polarity due both to their relative insensitivity and to the complexity of the first band of absorption which presents in some solvents a well-resolved coarse structure. This structure disappears by interaction with acid or polar solvents and by protonation on the carbonyl group.
Introduction The key idea for the interpretation of merocyanines’ solvatechromism was proposed independently by Kiprianovl and by B r o ~ k e r . ~According .~ to these authors, merocyanins such as I and 11are resonance hybrids betwen uncharged ‘a” and dipolar ‘b” forms.
’*Institute Universidad A u t h o m a de Madrid. de Quimica Maica.
When the solvent polarity increases, the contribution of charged forms, Ib and IIb, also increases. The solvation of So should be (1)Kiprianov, A. I.; Petrun’kin, V. E. J . Gen. Chem. USSR 1940,10,613. Kiprianov, A. I.; Timoschenko, E. S. J . Gen. Chem. USSR 1947,17, 1468. (2) Brooker, L. G. S.; Keyes, G . H.; Sprague, R. H.; Van Dyke, R. H.; Vandare, E.; Vanzandt, G.; White, F. L.; Cressman, H. W. J.; Dent, S. G. J . Am. Chem. SOC.1951,73,5332. (3) Brooker, L.G. S.;Keyes, G. H.; Heseltine, D. W. J . Am. Chem. SOC. 1951,73,5350.
0022-3654/92/2096-3615$03.00/0 0 1992 American Chemical Society
