Yung-Ching Chou

I. Three-center versus four-center HCl-elimination in photolysis of vinyl chloride at 193 nm: Bimodal rotational distribution of HCl (v<=7) detected with time-resolved Fourier-transform spectroscopy

Following photodissociation of vinyl chloride at 193 nm, fully resolved vibration-rotational emission spectra of HCl in the spectral region 2000–3310 cm−1 are temporally resolved with a step-scan Fourier-transform spectrometer. Under improved resolution and sensitivity, emission from HCl up to v=7 is observed, with J>32 (limited by overlap at the band head) for v=1–3. All vibrational levels show bimodal rotational distribution with one component corresponding to ∼500 K and another corresponding to ∼9500 K for v⩽4. Vibrational distributions of HCl for both components are determined; the low-J component exhibits inverted vibrational population of HCl. Statistical models are suitable for three-center (α, α) elimination of HCl because of the loose transition state and a small exit barrier for this channel; predicted internal energy distributions of HCl are consistent but slightly less than those observed for the high-J component. Impulse models considering geometries and displacement vectors of transition sta...

Three-center versus four-center elimination in photolysis of vinyl fluoride and vinyl bromide at 193 nm: Bimodal rotational distribution of HF and HBr (v⩽5) detected with time-resolved Fourier transform spectroscopy

Following photodissociation of vinyl fluoride (CH2CHF) and vinyl bromide (CH2CHBr) at 193 nm, fully resolved vibration–rotational emission spectra of HF and HBr in spectral regions 3050–4900 and 2000–2900 cm−1, respectively, are temporally resolved with a step–scan Fourier transform spectrometer. With a data acquisition window 0–5 μs suitable for spectra with satisfactory ratio of signal-to-noise, emission from HX (with X = F or Br) up to v=6 is observed. All vibrational levels show bimodal rotational distributions. For CH2CHF, these two components of HF have average rotational energies ∼2 and 23 kJ mol−1 and vibrational energies ∼83 and 78 kJ mol−1, respectively; the values are corrected for small quenching effects. For CH2CHBr, these two components of HBr correspond to average rotational energies ∼4 and 40 kJ mol−1, respectively, and similar vibrational energies ∼68 kJ mol−1. The separate statistical ensemble (SSE) model is suitable for three-center (α, α) elimination of HX because of the loose transiti...

Anomalous splittings of torsional sublevels induced by the aldehyde inversion motion in the S1 state of acetaldehyde.

The G6 group-theoretical high-barrier formalism developed previously for internally rotating and inverting CH3NHD is used to interpret the abnormal torsional splittings in the S1 state of acetaldehyde for levels 14(0-)15(0), 14(0-)15(1), and 14(0-)15(2), where 14(0-) denotes the upper inversion tunneling component of the aldehyde hydrogen and 15 denotes the methyl torsional vibration. This formalism, derived using an extended permutation-inversion group G6m, treats simultaneously methyl torsional tunneling, aldehyde-hydrogen inversion tunneling and overall rotation. Fits to the rotational states of the four pairs of inversion-torsion vibrational levels (14(0+)15(0A,E), 14(0-)15(0A,E)), (14(0+)15(1A,E), 14(0-)15(1A,E)), (14(0+)15(2A,E), 14(0-)15(2A,E)), and (14(0+)15(3A,E), 14(0-)15(3A,E)) are performed, giving root-mean-square deviations of 0.003, 0.004, 0.004, and 0.004 cm(-1), respectively, which are nearly equal to the experimental uncertainty of 0.003 cm(-1). For torsional levels lying near the top of the torsional barrier, this theoretical model, after including higher-order terms, provides satisfactory fits to the experimental data. The partially anomalous K-doublet structure of the S1 state, which deviates from that in a simple torsion-rotation molecule, is fitted using this formalism and is shown to arise from coupling of torsion and rotation motion with the aldehyde-hydrogen inversion.

Rotationally resolved spectra of transitions involving motion of the methyl group of acetaldehyde in the system à 1A″−X̃ 1A′

Fluorescence excitation spectra, at resolution 0.02 cm−1, in the system A 1A″−X 1A′ were recorded for acetaldehyde in a supersonic jet. We performed full rotational analysis of bands 1400+150n and 1400−150n, for n=0–4, in which 140+ and 140− denote the two inversion tunneling components of the aldehyde hydrogen out of plane bending, in the vibrational ground state of A 1A″. Torsional levels from near the methyl torsional barrier to beyond that barrier are assigned. Sublevels of torsional symmetry A below the barrier are fitted as an asymmetric rotor, but the resulting values of the rotational parameter A are affected significantly by the torsional motion. For the E sublevels, K doublet states split significantly with torsional quantum number vt. Anomalous transitions (ΔKa=0, ΔKc=0) to A sublevels are observed for bands 1400+1504 and 1400−1503, which cannot be simply explained from the Coriolis-type interaction of torsion and rotation. The positions of A and E sublevels in 140−15n cannot be fitted with a ...

Rotationally resolved spectra of transitions involving methyl torsion and C–C–O bend of acetaldehyde in the system of Ã1A″–X̃1A′

In the fluorescence excitation spectrum of acetaldehyde cooled in a supersonic jet, we performed a full rotational analysis of combination bands 10011400+150n and 10011400−150n, n=0–4 in the system A1A″–X1A′. The vibrational frequency of the C–C–O bending mode is determined to be ν10′=373.163(3) cm−1. The rotational structures of combination bands 10011400+1502, 10011400−1502, 10011400+1503, and 10011400−1504 resemble the structures of 1400+1502, 1400−1502, 1400+1503, and 1400−1504, respectively, but the intense E lines observed for 1400−1503 are not found in 10011400−1503. Torsional spacings observed in the C–C–O bend series are slightly smaller than those in the pure torsional series 140+ and 140−; these result from a decreased torsional barrier due to the C–C–O bending motion. Inversion spacings exhibit a pattern similar to those in the series 140+ and 140−. Reversed abnormal torsional sublevel A/E splittings are found for states n=0–2 of the 101140− series, similar to those in the 140− series. For st...

Rotationally resolved structures in the fifth and sixth torsional states of à 1A″ acetaldehyde: Internal rotation above the torsional barrier

The fluorescence excitation spectrum of acetaldehyde in its transition A 1A″–X 1A′ is analyzed for torsional states above the barrier. States with torsional vibrational quantum numbers vt=5A, 5E, and 6A at term energies 660–927 cm−1 are assigned. This region is 100–370 cm−1 above the torsional barrier. These states lie between the limits of torsional vibrational motion and free internal rotor motion, so that the close-lying 5A2 and 6A1 states mix for K>0, and K states in the E sublevel are widely split. From an analysis of calculated eigenfunctions, the composition of K=1 sublevels reaches nearly equal proportions of odd (A2) and even (A1) torsional wave functions when levels exceed the top of the torsional barrier, whereas the K=1 eigenfunctions of vt=4A1 lying near consists of mostly even (A1) functions. Avoided crossings with ΔK=0 and Δm=0 mod 3 selection rules are observed between K=2, E sublevels of 140+154 and 140+155 and between K=2, E sublevels of 140−154 and 140−155. The K energy-level structure...