ARTICLE pubs.acs.org/JPCA
Vibrationally Resolved Photofragment Translational Spectroscopy of CH3I from 277 to 304 nm with Increasing Effect of the Hot Band Min Cheng,† Zijun Yu,† Lili Hu,† Dan Yu,† Changwu Dong,† Yikui Du,* and Qihe Zhu* Beijing National Laboratory of Molecular Sciences, State Key Laboratory of Molecular Reaction Dynamics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China ABSTRACT: The photodissociation dynamics of CH3I from 277 to 304 nm is studied with our mini-TOF photofragment translational spectrometer. A single laser beam is used for both photodissociation of CH3I and REMPI detection of iodine. Many resolved peaks in each photofragment translational spectrum reveal the vibrational states of the CH3 fragment. There are some extra peaks showing the existence of the hot-band states of CH3I. After careful simulation with consideration of the hot-band effect, the distribution of vibrational states of the CH3 fragment is determined. The fraction σ of photofragments produced from the hot-band CH3I varies from 0.07 at 277.38 nm to 0.40 at 304.02 nm in the I* channel and from 0.05 at 277.87 nm to 0.16 at 304.67 nm in the I channel. Eint/Eavl of photofragments from ground-state CH3I remains at about 0.03 in the I* channel for all four wavelengths, but Eint/Eavl decreases from 0.09 at 277.87 nm to 0.06 at 304.67 nm in the I channel. From the ground-state CH3I, the quantum yield Φ(I*) is determined to be 0.59 at 277 nm and 0.05 at 304 nm. The curve-crossing probability Pcc from the hot-band CH3I is lower than that from the ground-state CH3I. The potential energy at the curve-crossing point is determined to be 32 740 cm-1.
1. INTRODUCTION CH3I, as a typical quasi-linear triatomic molecule for the photodissociation dynamics of polyatomic molecules, has been studied extensively both experimentally1-24 and theoretically25-38 in the past few decades. In the A-band absorption, the dissociation channels of CH3I are given as follows CH3 Iþhv f CH3 þI 2 P 1=2 ðI* channelÞ fCH3 þI
2
P3=2
ðI channelÞ
The UV absorption spectrum of CH3I in the A band (210350 nm) has been studied by Gedanken et al.39 and McGlynn’s group.40 In the A-band excitation (n f σ*), three electronic transitions 1Q1, 3Q0, and 3Q1 were reported.39 The absorption spectra of the transitions 1Q1, 3Q0, and 3Q1 show their maxima at 240, 261, and 300 nm,39 respectively. The latest calculations by Alekseyev et al.38 gave the absorption spectra for 1Q1, 3Q0, and 3Q1 transitions with maxima at 237, 257, and 279 nm, respectively. The parallel transition 3Q0 correlates to the I* channel, and the perpendicular transitions 1Q1 and 3Q1 correlate to the I channel. An avoided crossing is expected between the 3Q0 and the 1Q1 repulsive states as described by a simple Landau-Zener model.41 Earlier experimental studies on the photodissociation dynamics of CH3I at 2662 and 2484,5 nm reported that the CH3 fragments in the I* channel were populated in vibrational states of r 2011 American Chemical Society
the umbrella mode (ν2) with the maximum at ν2 = 2. However, the vibrational peaks were not resolved in their photofragment translational spectra. Subsequent experimental studies with resolved vibrational peaks showed the maximum population to be at the ground state (ν2 = 0) of the CH3 fragment in the I* channel at 266,10,23 248,7 and 240 nm.23 Similar results were obtained from IR studies24 and theoretical calculations.28-30,32 Parker’s group23 reported the hot-band effect from vibrationally excited states (ν30 , C-I stretch) of CH3I. Li et al. performed highresolution photofragment translational spectroscopy on CH3I. Near 304 nm,21 they found apparent extra peaks coming from the hot-band states of CH3I in the spectra of the I* channel but very weak extra peaks in the I channel. At 266 nm,20 they did not find the hot-band effect in both channels. Recently, Banares’s group42 studied the photofragment translational spectroscopy of CH3I near the red edge of the A band. From 304 nm to 333 nm, they found a very intense distribution from the hot-band states of CH3I in both channels but no hot-band effect at 286 nm. In comparison, the results of the hot-band effect are quite different between Li et al.21 and Banares’s group.42 At 304 nm, the distribution from the hot band in Banares’s result is much more intense than Li et al.’s result.21 At longer wavelengths, the distribution from CH3I (ν30 = 1) in Banares’s result is sometimes even much higher than that from the ground-state CH3I. Moreover, it is not Received: July 16, 2010 Revised: November 29, 2010 Published: January 27, 2011 1153
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clear from what wavelength the hot-band effect will become appreciable. It is worthwhile to study carefully the hot-band effect in the region from 277 to 304 nm with our high-resolution miniTOF photofragment translational spectrometer.
2. EXPERIMENTAL SETUP The experimental setup has been described elsewhere.43,44 A pulsed valve (General Valve, 0.5 mm diameter orifice) is operated at 10 Hz with a pulse width of about 250 μs. The sample CH3I (Sinopharm, Ltd., 99%) is used directly without further purification. The carrier gas Xe at 1 atm is bubbled through the liquid CH3I, and the gas mixture passing through the nozzle of the pulsed valve expands supersonically in the source chamber. The cooled gas expansion is collimated by a skimmer (d = 0.5 mm, 20 mm from the nozzle) to form a molecular beam in the reaction chamber. During operation of the pulsed valve the pressure of the source chamber increases from 1 10-5 to 6 10-3 Pa, but the pressure of the reaction chamber does not change much at about 1 10-5 Pa because of the small diameter (0.5 mm) of the skimmer. The molecular beam is crossed perpendicularly with a UV laser (about 0.05 mJ/pulse), which is the second harmonic of a dye laser (Spectra-Physics, Sirah) pumped by a Nd:YAG laser (Quanta-Ray, Pro 230) at a repetition rate of 10 Hz. In this work, the UV laser is used for both dissociation of CH3I and resonance-enhanced multiphoton ionization (REMPI) of the photofragment I* or I. The fragment ions Iþ are accelerated in the weak electric field region (E ≈ 1 V/cm, l1 = 17 mm) and then fly in the field-free region (L2 = 30 mm). The total flight path is only about 50 mm. The diameter of the screened hole on the detection plate is d = 6 mm. In the detection region, we put an extra grid plate of low voltage (-25 V) between the grounded detection plate and the high-voltage (-2 kV) microchannel plate (MCP) in order to prevent the strong field from penetrating into the field-free region. The signals from the MCP are recorded by a 500 MHz multichannel scaler (Fast, P7888-1E). The time sequence of the pulsed valve, the Nd:YAG laser, and the multichannel scaler is controlled by a digital delay/pulse generator (Stanford Research Systems, DG535). 3. EXPERIMENTAL RESULTS The TOF spectra of iodine ions with vibrational states resolved are obtained in both channels near 277, 280, 297, and 304 nm. For example, the typical TOF spectra of the I* channel at 277.38 nm with R = 0 and 90 (where R is the angle between the laser polarization and the detector axis; R = 0 correlates to the parallel transition, and R = 90 correlates to the perpendicular transition) are shown in Figure 1. The groups of the early and late peaks come from the I* fragments having a center-of-mass velocity (VCM) with a similar magnitude but with the direction forward and backward to the detector, respectively. The photofragment translational spectra (PTS) are transformed from the early peaks of the TOF spectra with the following formulas using the method as described in Xu et al.’s paper43 t qE t VCM ¼ a ¼ 2 mI 2 1 mR þmI 2 Et ¼ mI VCM 2 mR
Figure 1. TOF spectrum of the I* fragment at 277.38 nm: R = 0 with respect to parallel polarization and R = 90 with respect to perpendicular polarization.
where a is the acceleration of Iþ in the accelerating region, q is the unit charge, E is the electric field intensity, t is the turn-around time measured by the early and late peaks in the TOF spectra, and m is the mass of the CH3 or I fragments. The variations of other effects, such as the equivalent solid angle with different VCM, have also been considered. 3.1. Vibrational Distribution of CH3 Fragments in the I* Channel. At 277.38, 281.73, 295.91, and 304.02 nm, the vibrationally resolved photofragment translational spectra of CH3I photodissociation in the I* channel are shown in Figure 2. In each spectrum, there are several resolved peaks showing the umbrella vibration ν2 states of CH3 fragments with the highest peak at ν2 = 0. The vibrational energies for the ν2 mode of the CH3 fragment are 606, 682, and 731 cm-1 for 1 r 0, 2 r 1, and 3 r 2, respectively, from IR data.45 Excitation of ν1 (symmetric stretch of the CH3 radical, ∼3004 cm-1)46 is not observed in the I* channel. However, there exist extra peaks at the higher Et side of the ν2 = 0 peak in the spectra, showing the extra energy from the hot-band states ν30 = 1, 2 (C-I stretch, 528 cm-1)47 of CH3I. Our hot-band peaks at 304.02 nm are weaker than Li et al.’s21 at 1 atm. The difference is probably due to the better cooling in our supersonic molecular beam. In the spectra at 277.38 and 281.73 nm, the hot-band effect is relatively weak. It would be nice to be able to resolve the peaks from the hotband CH3I and those from the ground-state CH3I, as in the photofragment translational spectra of CF3I.48 Unfortunately, because of the small energy difference between the C-I stretch mode (ν30 = 528 cm-1)47 of CH3I and the umbrella mode (ν2 = 606 cm-1)45 of CH3, the peaks of ν2 = 1 and 2 of CH3 from the hot-band ν30 = 1 of CH3I will nearly overlap with the peaks of ν2 = 0 and 1 of CH3 from the ground-state ν30 = 0 of CH3I. Li et al.21 stated that the CH3 fragments from hot-band CH3I molecules were distributed in lower vibrational states than those from ground-state CH3I, but their experimental results could not show this statement. The theoretical calculations of Guo et al.49 and Amatatsu et al.30 showed that the vibrational distribution of the CH3 fragments from the hot-band state CH3I or the groundstate CH3I would not be very different. In addition, for the photodissociation of CF3I, the ν2 state distribution of CF3 fragments from the hot-band CF3I molecules did not show any 1154
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Figure 2. Photofragment translational spectra of the I* channel. The open circles are the experimental data, and the solid lines are the summed data of fitting components. The ν2 mode is the umbrella mode of CH3, while the ν30 mode is the stretch mode of C-I in CH3I. The labeled intensities are normalized from the detected signals not the real quantum yield because of the different REMPI cross section at different wavelengths.
lowering of the ν2 distribution from ground-state CF3I at 304.02 nm.48 Therefore, we use a similar relative ν2 distribution of the CH3 fragments (i.e., same Eint) from hot-band CH3I and ground-state CH3I in the simulation. The Gaussian profile is used to fit the peaks in the spectra with a typical fwhm of ∼400 cm-1. In the I* channel, the simulated vibrational distributions of CH3 from ground-state CH3I at 277.38, 281.73, 295.91, and 304.02 nm are given in Table 1. When the photon energy decreases, Eavl decreases, the measured internal energy of CH3 fragment decreases, but the Eint/Eavl does not change much at about 0.03 for all four wavelengths. Getting the integrated intensities I|| and I^ of the spectra at R = 0 and 90 in the same condition of the molecular beam and the same laser intensity, the anisotropy parameter β can be determined by the following equation )
I -I^ 0:5I þI^ )
β¼
The anisotropy parameters β, summarized in Table 1, are close to the limiting value 2 for the four wavelengths, indicating the parallel transition to 3Q0.
The simulated vibrational populations of CH3 fragments in the I* channel from hot-band state CH3I (ν30 = 1) are summarized in Table 3. The simulations are based on the similar ν2 distribution of CH3 from ν30 = 1 and 0 CH3I. As only ν2 = 0, 1 of CH3 from ν30 = 1 of CH3I are considered, Eint/Eavl become lower than those from the ground-state CH3I. The distribution from the hot-band CH3I is not intense in the spectra at 277.38 and 281.73 nm, but there is a clear and intense peak of CH3 (ν2 = 0) from hot-band CH3I (ν30 = 1) at 295.91 and 304.02 nm. The total fraction σ of distribution from the hotband CH3I increases violently from 0.07 at 277.38 nm to 0.40 at 304.02 nm (Table 5). The β of I* channel from hot-band CH3I is also close to 2. 3.2. Vibrational Distribution of CH3 Fragments in the I channel. The vibrationally resolved photofragment translational spectra of CH3I in the I channel at 277.87, 279.71, 298.23, and 304.67 nm are shown in Figure 3. The three high peaks in each spectrum reveal the ν2 = 0, 1, and 2 umbrella states of CH3 fragments from ground-state CH3I. There are also some low peaks for symmetric stretch ν1 = 1 combined with umbrella ν2 = 0, 1, and 2 of CH3 fragments. The hot-band effect is less 1155
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intense in the I channel than that in the I* channel. In simulation at each wavelength, the relative populations of ν2 = 0, 1, and 2 of CH3 from the hot-band CH3I are considered to be similar to those from the ground-state CH3I. The simulated vibrational populations of CH3 fragments in the I channel produced from the ground-state CH3I and from the hot-band state CH3I (ν30 = 1) are summarized in Tables 2 and 4. From CH3I (ν30 = 1), only ν2 = 0, 1, and 2 at ν1 = 0 of CH3 are considered in simulation, so Eint/Eavl becomes lower than that from the ground-state CH3I. (1) At 277.87 and 279.71 nm, the spectra in the I channel give the highest peak at ν2 = 1. The distributions of the symmetric stretch mode ν1 = 1 of CH3 are observed
appreciably at these wavelengths (see Figure 3a and 3b). After simulation, the vibrational distributions of CH3 from the ground-state CH3I are obtained and shown in Table 2. The distribution of the symmetric stretch ν1 mode is (ν1 = 0)/(ν1 = 1) = 0.80/0.20 at 277.87 nm and (ν1 = 0)/(ν1 = 1) = 0.78/0.22 at 279.71 nm. The Eint/Eavl is determined to be about 0.09 at the two wavelengths, higher than that in the I* channel. In Figure 3a and 3b, the right edge of the ν2 = 0 peak is slightly broader due to the hot band of CH3I. The fraction σ from the hot-band CH3I is determined to be about 0.05 and 0.06, lower than σ = 0.07 and 0.11 in the I* channel at 277.38 and 281.73 nm, respectively. The value of β is determined to be 1.91and 1.92, showing that the I channel comes via a parallel transition to 3Q0 and then crossing to 1Q1. (2) At 298.23 nm, the highest peak changes from ν2 = 1 to ν2 = 0 as shown in Figure 3c, because of the decrease of Ehν. The vibrational distribution of CH3 from groundstate CH3I is shown in Table 2. The distribution of the ν1 mode is (ν1 = 0)/(ν1 = 1) = 0.89/0.11. The fraction σ from hot-band CH3I is determined to be 0.13. The value of β is determined to be 1.70, which means the I fragments are produced dominantly via a parallel transition 3Q0 and then crossing to 1Q1, with a small portion produced from perpendicular transition 3Q1. (3) At 304.67 nm, the spectra at R = 0 and 90 have the highest peak at ν2 = 0, as shown in Figure 3d and 3e. From the ground-state CH3I, the vibrational distributions of CH3 are given in Table 2. The ν1 mode distributions are obtained with (ν1 = 0)/(ν1 = 1) = 0.94/0.06 at R = 0 and (ν1 = 0)/(ν1 = 1) = 0.97/0.03 at R = 90. Eint/Eavl is around 0.06 at R = 0 and 90, lower than those at shorter
Table 1. Vibrational Population of CH3 Fragments, Eint/Eavl, Anisotropy Parameter β, and I* Quantum Yield Φ(I*) in the I* Channel Produced from Ground-State CH3I (ν30 = 0) ν2 λ (nm)
0
1
β
Φ(I*)
0.021
1.93
0.73
0.028
1.9
0.68
23
0.76
19
2
3
Eint/Eavl
0.004
0.020
ref
248
0.66 0.26
0.08
248
0.65 0.29
0.06
266
0.63 0.28
0.065
266
0.72 0.25
0.03
266
0.64 0.31
0.04
0.025
1.95
0.69
20
277.38
0.68 0.27
0.05
0.028
1.93
0.59
this work
281.73 295.91
0.69 0.25 0.71 0.25
0.06 0.04
0.029 0.030
1.92 1.92
304.02
0.76 0.20
0.04
0.029
1.88
0.05
this work
304.02
0.85 0.15
