ARTICLE pubs.acs.org/JPCA
Photoelectron Imaging Spectroscopy of S1(1B2u π,π*) Benzene via 611n (n = 0-3) Levels Dongmei Niu,†,§ Yoshihiro Ogi,† Yoshi-Ichi Suzuki,†,‡ and Toshinori Suzuki*,†,‡ † ‡
Chemical Dynamics Laboratory, RIKEN Advanced Science Institute, RIKEN, Wako 351-0198 Japan Department of Chemistry, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan
bS Supporting Information ABSTRACT: We report resonance-enhanced two-photon ionization photoelectron spectroscopy of jet-cooled benzene via the 611n (n = 0-3) vibronic levels in S1(1B2u π,π*) using a nanosecond UV laser and photoelectron imaging. The best energy resolution (ΔE/E) was 0.7%. The photoelectron spectrum from the S1 6113 level (Evib = 3284 cm-1) in the channel three region exhibited a clear signature of intramolecular vibrational redistribution (IVR). The spectral features were consistent with picosecond zero kinetic energy photoelectron (ZEKE) spectra reported by Smith et al. [J. Phys. Chem. 1995, 99, 1768]. The photoelectron angular anisotropy parameter β2 was found to be negative in ionization from the 611n (n = 0-3) levels with photoelectron kinetic energies up to 5000 cm-1. No influence of a shape resonance was identified.
’ INTRODUCTION Benzene is the most fundamental aromatic molecule, and considerable numbers of studies have been reported. The first excited singlet state (S1 1B2u) of benzene is the best-known example of a vibronically induced forbidden transition, and photophysics of the S1 state has received great attention over the years. Parmenter and Schuyler have shown that the fluorescence quantum yields (ΦF) of low-lying S1 vibronic levels such as 00 and 61 are only ∼0.2, indicating the presence of nonradiative decay.1 (We follow the vibrational mode numbering by Wilson and use the superscript to express the vibrational quantum number in S1.) Spears and Rice investigated the vibronic state dependence of ΦF more thoroughly and observed that ΦF diminishes specifically by excitation of out-of-plane vibrations.2 The state dependence was confirmed further under collision-free conditions by Stephenson and Rice in the 1980s.3,4 The results were explained by radiationless transition theory for weak coupling of electronic states. The primary nonradiative deactivation pathway from S1 benzene was concluded to be intersystem crossing (ISC). Following the work by Spears and Rice, Callomon et al. discovered abrupt absorption line broadening at around Evib = 3000 cm-1 in S1; they claimed that the radiationless transition rate suddenly increases by 3 orders of magnitude to be 1011 s-1.5 The sudden increase of the deactivation rate was ascribed to an unknown deactivation pathway that seemed to open at around Evib = 3000 cm-1. This new channel was termed the “channel three” as fluorescence and ISC were already known as two other channels. The channel three exhibited an interesting vibronic state dependence; the line broadening was less extensive for the series of 711n than that for 611n. As shown in Figure 1, the normal mode 7 is primarily the C—H stretching mode. r 2011 American Chemical Society
Despite extensive research over the years, the nature of the channel three has not been completely elucidated. In the following, presented is a brief summary, by no means a complete review owing to limited space, of important findings regarding the channel three. Sumitani, Yoshihara, and co-workers have performed ultrafast fluorescence spectroscopy of benzene vapor and speculated that the channel three is photochemical isomerization from S1 via unknown electronic state X.6,7 Riedle, Neusser, and Schlag have shown that intramolecular vibrational redistribution (IVR) is involved in the dynamics of the channel three by Doppler-free two-photon absorption spectroscopy.8 Achiba, Hiraya, and Kimura performed resonance-enhanced multiphoton ionization (REMPI) photoelectron spectroscopy of the 611n series using a nanosecond laser and observed the vibronic bands by REMPI with intensities almost proportional to their absorption strengths.9 From this fact, they suggested that virtually no electronic quenching or chemical reaction occurs from these levels in S1 in the time-window (10-100 ps) of photoionization and that only IVR is operative in this short time range. To clarify the mechanism of the channel three, the lifetimes of vibronic states should be measured accurately. Smith, Zhang, and Knee (SZK) performed picosecond PFI-ZEKE (pulsed field ionization zero kinetic energy) photoelectron spectroscopy in a molecular beam.10 They found the lifetimes of 6113, 7111, and 6114 to be 285, 115, and 70 ps, respectively. (These lifetimes were approximate as they exhibited nonexponential behaviors.) These values are consistent with the fluorescence lifetimes determined for benzene vapor by Sumitani et al.6 as well as the IVR time scale Received: November 4, 2010 Revised: January 14, 2011 Published: March 02, 2011 2096
dx.doi.org/10.1021/jp110557n | J. Phys. Chem. A 2011, 115, 2096–2102
The Journal of Physical Chemistry A
Figure 1. The normal modes of benzene, obtained by an ab initio calculation (MP2/4-31g).
estimated by Achiba et al.9 These lifetimes are also much shorter than the lifetime (>1 ns) of the 71 vibronic state. Smith et al. have estimated the time constant of IVR from the 6113 state to be 3035 ps.10 The IVR time constant seemed longer for 7111, although it was not quantified. Such mode specificity is consistent with the observation by Callomon et al.5 Kato has performed pioneering theoretical work by MCSCF and MCSCF-CI methods on deactivation of S1 benzene and concluded that the channel three involves internal conversion (IC) to S0 and photoisomerization reaction to prefulvene; the S1 potential energy surface crosses with S0 in the nonplanar distorted geometry, and it is adiabatically connected to the prefulvenic isomer.11 Kato’s estimate of the deactivation time constants agreed well with those measured by Sumitani et al.6 (tens of ps to 100 ps), although Kato assumed that IVR was complete prior to IC and isomerization (RRKM behavior). Kato himself pointed out that more detailed investigation of IVR is necessary for accurate evaluation of the deactivation rates.11 This was clear as there was a mode specificity in the channel three decay. Later, a number of theoretical works12-23 were reported on the excitedstate potential energy surfaces and dynamics of benzene, which have shown the existence of a conical intersection between the potential energy surfaces of S1 and S0 involving prefulvenic isomer, and that it is the primary driving force of ultrafast IC. The line broadening observed by Callomon et al.5 in the channel three region is caused by coupling of the optically bright state with the dark states, in which the counterpart dark states are most likely of S1 or S0. Suzuki and Ito observed the dispersed fluorescence spectra from 71 and 6113 levels with a resolution of 6-12 cm-1 and showed that 71 has no signature of IVR whereas 6113 exhibits a broad spectrum indicative of IVR.24 They estimated that the IVR was more than 2 orders of magnitude greater from 6113 than that from 71. Thus, IVR in the channel three region is incomplete prior to IC and photochemical reaction, which implies that incomplete IVR should be taken into account for theoretical simulation of the channel three decay, as mentioned above. For examining the state-dependent IVR, it is ideal to perform time-resolved spectroscopy for various vibronic states in the channel three region. Dispersed fluorescence spectroscopy is a classic technique to study IVR; however, as the observation time window (i.e., fluorescence lifetime) in fluorescence spectroscopy varies with a vibronic state; Suzuki and Ito have taken into account the different fluorescence lifetimes.24 To time-gate dispersed fluorescence spectroscopy, Parmenter has used collisional quenching
ARTICLE
of S1 benzene by molecular oxygen. This so-called chemical timing25,26 method was a very intersting method, but its accuracy was limited owing to strong perturbation caused by molecular collisions. Time-resolved photoelectron spectroscopy (TRPES) or fluorescence up-conversion methods allow considerably shorter time-gates only limited by a laser pulse duration. Picosecond ZEKE spectroscopy by Smith et al. has been the most sophisticated experiments performed so far on the channel three of benzene.10 However, the ZEKE spectra were measured for very limited energy ranges. The first REMPI-PES (the photoelectron spectroscopy) of benzene was reported by Long, Meek, and Reilly in 1983;27 they observed clearly resolved photoelectron spectra with two-photon ionization via the 61, 6111, and other low-lying levels in S1. Achiba et al. have performed REMPI-PES via higher vibronic levels in the 611n series and argued that the photoelectron spectra become broader upon going to higher vibronic states.9 One of the highest-resolution photoelectron spectra of benzene was by Lindner et al. using the ZEKE method,28 in which rotational resolution was achieved. A similar vibrationally resolved spectrum was also observed by Burrill et al. using MATI (mass-analyzed threshold ionization).29 However, both of these were for 61. In our previous work, we demonstrated super-resolution photoelectron imaging for 61 and 6111 levels.30 In this work, we extended our measurement up to the 6113 level and examined IVR by photoelectron spectroscopy. Our experiment reported here shares the spirit with that of Achiba et al. reported nearly 30 years ago,9 although the photoelectron kinetic energy resolution is considerably improved.
’ EXPERIMENTAL SECTION (a). Molecular Beam Apparatus. Helium gas was flowed over benzene liquid in a sample cell held at room temperature, and the mixed gas was expanded from a pulsed valve at a repetition rate of 25 Hz into a source chamber. The stagnation pressure of the gas was 2 atm. The pressure in the source chamber, evacuated with a 1400 L/s turbo molecular pump, was 3 10-5 Torr when running the pulsed valve. The supersonic gas jet was sampled through a skimmer of 0.5 mm orifice diameter into a buffer chamber evacuated by a 200 L/s turbo molecular pump. The supersonic molecular beam thus generated was further collimated with another skimmer of 0.5 mm orifice diameter and introduced into a photoionization chamber evacuated by a 500 L/s turbo molecular pump. The pressure in the photoionization chamber was 3 10-8 Torr. The molecular beam traveled toward the microchannel plate (MCP) detector. Benzene molecules in the molecular beam were ionized by a pulsed UV laser in the middle of an electric field for acceleration of electrons, and the spatial distribution of photoelectrons was projected onto the MCP. The rotational temperature of the sample was not measured but estimated to be
