Changwu Dong

Vibrationally resolved photofragment translational spectroscopy of CH3I from 277 to 304 nm with increasing effect of the hot band.

The photodissociation dynamics of CH(3)I from 277 to 304 nm is studied with our mini-TOF photofragment translational spectrometer. A single laser beam is used for both photodissociation of CH(3)I and REMPI detection of iodine. Many resolved peaks in each photofragment translational spectrum reveal the vibrational states of the CH(3) fragment. There are some extra peaks showing the existence of the hot-band states of CH(3)I. After careful simulation with consideration of the hot-band effect, the distribution of vibrational states of the CH(3) fragment is determined. The fraction σ of photofragments produced from the hot-band CH(3)I 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 . E(int)/E(avl) of photofragments from ground-state CH(3)I remains at about 0.03 in the I* channel for all four wavelengths, but E(int)/E(avl) decreases from 0.09 at 277.87 nm to 0.06 at 304.67 nm in the I channel . From the ground-state CH(3)I, the quantum yield Φ(I*) is determined to be 0.59 at 277 nm and 0.05 at 304 nm. The curve-crossing probability P(cc) from the hot-band CH(3)I is lower than that from the ground-state CH(3)I. The potential energy at the curve-crossing point is determined to be 32,740 cm(-1).

Photofragment translational spectroscopy of CH3I at 225 nm—with the high excitation of the symmetric stretch vibration of CH3 fragment

The photodissociation dynamics of CH(3)I at 225 nm is studied on our high resolution mini-TOF photofragment translational spectrometer. The photofragment translational spectra of the I* and the I channels via parallel (∥) and perpendicular (⊥) transitions, i.e., of the four pathways (3)Q(0), (3)Q(0) ← (1)Q(1), (1)Q(1), and (1)Q(1) ← (3)Q(0), are obtained with both the symmetric stretch (ν(1)) and the umbrella (ν(2)) vibrational modes of the CH(3) fragments partially resolved. The strong excitation of the symmetric stretch mode (ν(1)) is revealed in both the I and the I* channels. The branching fractions for the four pathways (0.09 for (3)Q(0), 0.03 for (3)Q(0) ← (1)Q(1), 0.34 for (1)Q(1), and 0.54 for (1)Q(1) ← (3)Q(0)) show that the parallel transition ((3)Q(0) ← X) is the major and the I channel is dominant in the photodissociation of CH(3)I at 225 nm. The curve-crossing probability is found to be 0.86 for (1)Q(1) ← (3)Q(0) but 0.08 for (3)Q(0) ← (1)Q(1).

Vibrationally mediated photodissociation of CH3I [v1 = 1] at 277.5 nm: the vibrationally adiabatic process.

From the photofragment translational spectra of C-H symmetric stretch excited CH3I [v1 = 1, v2 = 0] photodissociatioin at 277.5 nm, the vibrational distribution of photofragments CH3 (v1 = 0, v2 = 0), (0,1), (1,0), (1,1) in the I* channel are measured to be 0.02, 0.02, 0.47, 0.25, and those of CH3 (1,0), (1,1) in the I channel are 0.04, 0.05, respectively. It shows that most of the dissociated CH3I [1,0] retain the C-H symmetric stretch vibration v1 = 1 in the photofragments CH3, and the vibrational distribution in umbrella bending mode is not seriously affected by the original C-H symmetric stretch excitation. The photodissociation of CH3I [1,0] mainly follows the vibrationally adiabatic process. The original vibrational excitation [v1 = 1] of CH3I is quite like a spectator, and the intramolecular vibrational-energy redistribution (IVR) does not play obvious part during photodissociation.

Rotamers and isotopomers of 3-chloro-5-fluoroanisole studied by resonant two-photon ionization spectroscopy and theoretical calculations.

The ab initio and density functional theory (DFT) calculations reveal that two rotamers, denoted by cis and trans 3-chloro-5-fluoroanisole (3C5FA), are stable for each of the S(0), S(1), and D(0) states. In the one-color resonant two-photon ionization (R2PI) spectra, the band origins of the S(1)←S(0) electronic transition (0(0) bands) of cis(35)Cl-3C5FA and cis(37)Cl-3C5FA are both located at 36,468 ± 3 cm(-1), while the 0(0) bands of trans(35)Cl-3C5FA and trans(37)Cl-3C5FA are found to be 36,351 ± 3 and 36,354 ± 3 cm(-1). The two rotamers display very similar vibrational frequencies in the S(1) state, and the observed active modes mainly involve the in-plane ring deformation vibrations. By the two-color R2PI spectroscopy, the adiabatic ionization energies (IEs) of both isotopomers of 3C5FA are determined to be 69,720 ± 15 cm(-1) for the cis rotamer and 69,636 ± 15 cm(-1) for the trans rotamer. The substitution, conformation, and isotope effects on the properties of 3C5FA, including the molecular structures, vibrational frequencies, and electronic transition and ionization energies, were also discussed in detail.

Resonance-enhanced two-photon ionization spectroscopy and theoretical calculations of 3,5-difluoroanisole and its Ar-containing complex

The structure and vibrations of 3,5-difluoroanisole (3,5-DFA) in the first electronically excited (S(1)) state were studied by mass-analyzed resonant two-photon ionization (R2PI) technique as well as the quantum chemical calculations. The ab initio and density functional theory (DFT) calculations reveal that only one structure is stable for each of the S(0), S(1), and D(0) states. In the one color R2PI spectrum, the band origin of the S(1)←S(0) electronic transition (0(0) band) of 3,5-DFA is found to be 37,595±3 cm(-1). In the S(1) state, most of the bands observed are related to the in-plane ring deformation and out-of-plane bending vibrations. The adiabatic ionization energy (IE) of 3,5-DFA is determined to be 70,096±15 cm(-1) by the two color R2PI technique, in agreement with the values predicted by the DFT approaches. The dihalogen-substitution effects on the molecular structure, vibrational frequencies, and electronic transition and ionization energies were discussed in detail. The van der Waals complex of 3,5-DFA with argon (3,5-DFA···Ar) was also observed and studied. The 0(0) band of 3,5-DFA···Ar complex is red-shifted by about 9 cm(-1) with respect to that of 3,5-DFA. Both the experimental data and the calculated results indicate that the formation of 3,5-DFA···Ar complex gives only a weak influence on the properties of 3,5-DFA moiety.

Photofragment translational spectroscopy at 304 nm from C-H symmetric stretch excited CH 3 I [ v 1 = 1]

The 304 nm photodissociation of the C-H symmetric stretch excited CH3I [v1 = 1, v2 = 0] (v1 denotes the C-H symmetric stretch mode, and v2 denotes the umbrella mode) is studied with our simple photofragment translational spectrometer. An IR laser is used to excite the ground state CH3I [0, 0] to the C-H symmetric stretch excited CH3I [1, 0]. With IR laser OFF and ON, the fractions of photofragments CH3 (ν1, ν2) from the 304 nm photodissociation of CH3I [1, 0] have been determined through the photofragment translational spectra (PTS) from measuring I and I* and also through the PTS from measuring CH3 (0, 0) (1, 0) (0, 1) and (1, 1). The experimental results show that the C-H symmetric stretch vibration (v1 = 1) in parent molecules is about 66% retained in the photofragments in the I channel, but only 24% in the I* channel. The populations of photofragments CH3 (0, 2) and (0, 3) are higher than CH3 (0, 0) and (0, 1), showing strong inverted population both in I and I* channels.