The laser induced predissociation dynamics of the B Rydberg state of CH3I following two-photon absorption of a pump pulse was studied with femtosecond pump-probe photoelectron imaging coupled with time-resolved mass spectroscopy. The predissociation lifetime was measured to be 1.55 ps induced by the crossing between the B state and the repulsive A-band. Two possible predissociation channels were observed originating from (a) direct coupling between the B state and the repulsive 3 Q 0 state and (b) a second crossing between the 3 Q 0 and 1 Q 1 states after the coupling between the B and 3 Q 0 states, respectively.
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The initiation and subsequent control or exploration study of chemical transformations in real time using ultrashort laser pulses is the aim of femtochemistry [1,2]. The real-time investigation of the ultrafast dynamics of molecules in gas and condensed phases has attracted a great deal of attention over the last two decades [3–6]. A significant number of studies have dealt with time-resolved molecular dynamics in systems of increasing complexity [2,7–12]. However, some basic issues of the real time dynamics are still elusive. New insight into the detailed mechanisms of some simple systems can be obtained by the combination of state-of-the-art femtosecond laser systems and detection techniques.
For the detailed study of chemical transformation dynamics in real time, time-resolved photoelectron imaging (TRPEI)  has proven to be a powerful technique. TRPEI measures the photoionization differential cross section, i.e. the energy resolved within the probe bandwidth. It thus provides both the energy and the angular distribution of the photoelectron as well as their correlation as a function of time. In particular, the sensitivity of the photoelectron angular distribution to the electronic symmetry translates into useful information regarding the dynamics of nonradiative transitions including internal conversions and intersystem crossing in excited electronic states.
Methyl iodide, as a prototype system with C3v symmetry, has been tested heavily with a variety of experimental and theoretical techniques over more than two decades [14–16]. The excited states of methyl iodide have long attracted considerable interest. Investigations of the dissociation occurring in the lowest excited dissociative states, the A band, and the predissociation of higher lying excited states to the A band have served as benchmark studies of the dissociation of an isolated molecule. With the development of femtosecond techniques, the relatively low number of atoms and the richness and complexity of dissociation or predissociation processes has also made methyl iodide a landmark system for real-time studies [16–18]. Most of these gas phase studies, however, have concentrated on the direct dissociation of the A-band. More recently, the spectroscopy and dynamics of the higher lying and predissociative Rydberg states have been attracting attention. The first Rydberg state near 200 nm, termed B, arises from electronic transitions from a 5p iodine orbital to a higher atomic Rydberg orbital 6s of the iodine atom  and exhibits a vibronically resolved structure in which the rotational structure is obscured due to lifetime broadening . For the A-band and the lower lying Rydberg levels, two available dissociation channels are CH3 + I(2P3/2) and CH3 + I*(2P1/2). One or more of the states contributing to the A-band is thought to intersect the B state and induce its predissociation [19,21]. During the predissociation process, some vibration modes have played important roles [21,22]. The modes involved predissociation and the state lifetimes of CH3I have been discussed using the resonance enhanced multiphoton ionization scheme , Raman excitation  or time-resolved mass spectrometry (TRMS) .
In this work, we have studied the laser induced predissociation dynamics of CH3I from B state to A-band in real time by combining the time-resolved mass spectrometry and TRPEI techniques. The possible detailed predissociation mechanism through curve crossing of the B Rydberg state with the steep repulsive potential of the unbound A-state surface is analyzed and described. The predissociation mechanism is assumed to account for the short lifetime of this state and determined the photoelectron release with different energies. The femtosecond pump-probe technique has allowed the real-time observation of the energy and the angular distribution of the photoelectron.
The time-resolved photoelectron imaging setup used in this study is similar in spirit to our ion velocity imaging system, which has been described elsewhere . The key modification was made by adding a multilayer μ-metal shielding to avoid disturbance of the stray field when collecting the photoelectrons. The details of our laser system were described elsewhere . Briefly, the seed beam was generated by a commercial Ti:sapphire oscillator pumped by a CW second harmonic of an Nd:YVO4 laser, and then amplified by an Nd:YLF pumped regenerative amplifier to generate a 1 kHz pulse train centered at 800 nm of 45 fs pulse width with maximum energy of 1 mJ/pulse. This fundamental light was split into two equal intensity beams. One beam was frequency doubled to ~400 nm with 6 nm bandwidth in a beta barium borate crystal (BBO type I) to act as the pump light. The other beam passed a step-motor-controlled optical delay stage as the probe light. The two beams with vertical polarization were merged by a dichroic mirror and directed into the molecular beam chamber. Methyl iodide, 5% seeded in helium buffer gas at a background pressure of 2 atm, was expanded through a pulsed valve to generate a pulsed molecular beam. The beam was skimmed and introduced into the ionization chamber where it was intersected perpendicularly by the laser beam. The CH3I molecules were populated to the B Rydberg state by two-photon absorption of the pump pulse and then ionized by multiphoton ionization of the probe light. The transformation process was investigated by detecting photoelectrons (TRPEI) or photoions (TRMS). Typical pulse energies in our work were about 7~8 μJ/pulse for the pump and about 30 μJ/pulse for the probe pulse. The cross-correlation was 50 ± 10 fs.
The mass spectra of CH3I obtained at the temporal overlap of the pump and the probe light have only two peaks corresponding to CH3I+ and CH3 +, respectively. The area ratio of CH3I+ to CH3 + is 9:1. Figure 1 shows the total ion signals, integrated over all recoil speeds, measured for CH3I+ and CH3 +, respectively, as a function of the pump-probe delay. Both the polarizations of the pump and the probe laser were vertical, and were perpendicular to the molecular beam. The observed decays for CH3I+ and CH3 + were fitted to the sum of a Gaussian (cross correlation function) and a single exponential decay convolved with an instrument function. The corresponding delay-time constants are yielded to be τ 1 = 1.55 ps and τ 2 = 1.09 ps, respectively. We believe that the CH3 + ions are from the dissociation of CH3I+ parent ions and have no contribution to the total photoelectron signal. This is discussed later.
Figure 2 shows typical photoelectron images measured at a series of delay times between the pump and probe pulses. The left parts of the images are the raw accumulated images and the right parts are the inverse Abel transforms of the observed data, representing a section of the 3D photoelectron scattering distribution. Six well-resolved concentric rings appear in the observed images with the delay time changing, which are obvious especially in the image acquired at 180 fs delay time. The rings with different radii stand for different photoelectron kinetic energy components. The corresponding photoelectron energy distributions of these images are shown in Fig. 3 . These six peaks center at 0.48, 0.66, 0.94, 1.12, 1.99 and 2.69 eV respectively, and are assigned to be peak 1, 2, 3, 4, 5 and 6 in order of increasing energy hereinafter.
We have shown the absolute ion counts of each peak in the raw images at different delays in Fig. 4 . The FWHMs of these six curves are 140, 260, 250, 131, 128 and 126 fs, respectively. It can be seen that the decay times of the ion counts for peaks 2 and 3 are obviously longer than for the other four. The longer decay times of these two peaks suggested that they may have a different production mechanism.
Peaks 1 and 4 can be explained by a (2 + 3) (two pump and three probe photons) ionization scheme via the B Rydberg state as shown in Fig. 5 . The molecules are excited to the B state after two-photon absorption of the pump pulse and then ionized by three probe photons. These two peaks correspond to CH3I+ in two spin-orbit states, (2 E 3/2) and (2 E 1/2) . The calculated available energies (maximum of the released kinetic energy) of this ionization scheme agree well with our experimental data of 0.48 and 1.12 eV. Similarly, Peaks 5 and 6 can be explained by a (3 + 2) (three pump and two probe photons) scheme via a higher Rydberg state (Fig. 5).
As we have predicted, peaks 2 and 3 have longer decay times than the other four. Also, the relative intensities of these two peaks became stronger with increasing delay time, while the relative intensities of the first four peaks did not change after 440 fs. All of these factors indicate a conversion between different intermediate states. The released kinetic energies of these two peaks are close to that of (2 + 3) scheme. Theoretical and experimental studies on surface crossing and predissociation dynamics have proved that the B Rydberg state has a surface crossing with the dissociative A-band [20,22,27]. The mode-specific subpicosecond predissociation dynamics of the methyl iodide B state has been investigated [20,22] and the lifetimes of different vibration modes involved in the predissociation have been measured. Considering the 6 nm bandwidth of our pump laser, the band origin 00 0, the C-I stretching mode 30 1 and CH3 asymmetric rock 60 1 are conceivably involved [22,28]. The lifetimes of these three states have been reported to be 1.38, 4.1 and 0.98 ps, respectively . The lifetime fitted for the time-dependent ion signals of CH3I+ is 1.55 ps, which may be a colligation lifetime of these three possible intermediate states. The measured lifetime of CH3 + is 1.09 ps, which is a little shorter than that of CH3I+. We believe that CH3 + ions come from the dissociation of parent CH3I+ ions, because the time-dependent ion signals of CH3I+ and CH3 + have the same trend and the total energy in the previously mentioned (3 + 2) scheme has exceeded the appearance potential of 12.18 eV  of CH3 + + I. If they are from the neutral dissociation of excited CH3I, seven probe photons are needed to ionize the neutral CH3 radical whose IP is 9.84 eV . This scheme is unfavorable in the energy view. Also, the neutral dissociation channel required to produce the CH3 radical is competitive with the ionization process to produce CH3I+. If in this case, the time-dependent ion signals of CH3I+ and CH3 + should have opposite trends. Thus, it is reasonable that CH3 + came from the dissociation of CH3I+. The lifetime difference between CH3I+ and CH3 + may come from different involved intermediate vibration modes. The dissociation of parent ions into CH3 + may only have correspondence with some special mode, while the appearance of CH3I+ is a colligation effect of several intermediate vibration modes. It is reasonable that peaks 2 and 3 are from the predissociation of the B state via the crossing with the repulsive A-band.
Calculated potential energy curves for the dissociative A states indicate that the A2 component of the 3 Q 0 state undergoes a crossing through the low energy regions of B2 and C2 and, therefore, is the most likely state to induce predissociation [28,30]. If only the 3 Q 0 state is involved in the predissociation, two peaks should appear at Ea<1.12eV and Eb<0.48eV. However, the two peaks that we have observed are 0.48 eV<Ea<Eb<1.12 eV. It is known that the A-band of CH3I is composed of three repulsive states, 3 Q 0, 1 Q 1 and 3 Q 1 as Mulliken assigned . 3 Q 0 and 1 Q 1 have a strong non-adiabatic coupling. If the probe channels are open below the crossing point of 3 Q 0 and 1 Q 1, there are two evolving predissociation paths below the crossing point as shown in Fig. 5. If these two ionization channels both converged to the lower spin-orbit state of IP, two peaks 0.48 eV<Ea<Eb<1.12 eV should be observed. This is consistent with what we have observed in our experiments. Therein, peak 3 was probed from the 3 Q 0 state, which was evolved on a higher energy surface and led to a larger available energy. Peak 2 was probed from the 1 Q 1 state, which is steeper. These predissociation processes are also shown in Fig. 5. The relative intensities of peaks 1, 2, 3 and 4 remained unchanged when the delay time was equal to or greater than 440 fs. That means that the conversion from the B state to the 3 Q 0 state has achieved a balance. The coupling time of the B and 3 Q 0 states was estimated to be less than 440 fs.
These rings in the photoelectron images have shown strong anisotropy. This is because of the nonbonding electron excitation of the iodine atom via either the B Rydberg state or the dissociative A-band when the methyl iodide is ionized. The final photoelectron angular distributions have memorized the atomic-like electron orbital in the intermediate state when the local nonbonding electron is removed to ionize. Strong anisotropy in the photoelectron images points to atomic-like electron orbitals in the intermediate states.
M. Tsubouchi et al.  have investigated the photodissociation of the NO dimer, and the photoelectron signal exhibited a time-dependent energy shift indicating that this signal was due to ionization of the dissociating dimer. The energy shift is only 10-20 meV. In our experiments, when the molecules are ionized after the crossing point between the B state and the A-band, there should be an energy shift when the wave packet evolves on the dissociative surfaces. However, no obvious energy shift has been observed. There are two possible reasons, the first being that our imaging resolution is about 20 meV at 1 eV. The limitation of the imaging resolution did not permit the obvious observation of the very small energy shift. Also, the lifetime of the A-band is too short. The energy will be lost very quickly in these repulsive states. The lost energy was used to break the C-I bond to produce neutral fragments. Zewail and associates measured the A-band lifetime using time-resolved kinetic energy time of flight mass spectrometry , and the reaction time was determined to be 125 fs. The energy surfaces of the 3 Q 0 and 1 Q 1 states are very steep. When we open the probe beam at longer pump-probe delay times, all of the molecules that are converted to A-band dissociation cannot be ionized by three probe photons anymore because of energy loss when the molecules are evolved on the steep repulsive surfaces.
We have demonstrated the use of TRPEI for studying the predissociation of the B Rydberg state in CH3I. The interesting predissociation process of the B Rydberg state was observed when the molecules were excited to the B Rydberg state by absorption of two pump photons and then ionized during the predissociation process induced by the surface crossing of the B state and the repulsive A-band. Two repulsive states, 3 Q 0 and 1 Q 1, are responsible for the two predissociation peaks.
This work was supported by the National Natural Science Foundation of China under Grant nos. 20703061, 20703060 and 20673140.
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