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Notes: High-resolution photoionization efficiency (PIE) and pulsed field ionization photoelectron (PFI-PE) spectra for CS2 have been measured using coherent vacuum ultraviolet (VUV) laser radiation in the energy range of 81 050–82 100 cm−1. The PIE and threshold photoelectron (TPE) spectra for CS2 in the energy range of 80 850–82 750 cm−1 have also been obtained using synchrotron radiation for comparison with results of the VUV laser study. The analysis of the PIE spectra reveals three Rydberg series converging to the excited CS2+(2Π1/2) spin–orbit state. These series, with quantum defects of 1.430, 1.616, and 0.053, are associated with the [2Π1/2]npσu, [2Π1/2]npπu, and [2Π1/2]nfu configurations, respectively. The Stark shift effect on the ionization threshold of CS2 has been examined as a function of dc electric fields (F) in the range of 0.65–1071 V/cm. The observed F dependence of the Stark shift for the ionization onset of CS2 is consistent with the prediction by the classical adiabatic field ionization formula. The extrapolation of the ionization onset to zero F yields accurate values for IE[CS2+(X˜ 2Π3/2)]. This study shows that in order to determine accurate IEs and to probe autoionizing structures for molecular species by PIE measurements, it is necessary to minimize the electric field used for ion extraction. The assignment of Renner–Teller structures resolved in the VUV PFI-PE spectrum is guided by the recent nonresonant two-photon (N2P) PFI-PE and theoretical studies. The analysis of the PFI-PE spectrum also yields accurate values for IE[CS2+(X˜ 2Π3/2,1/2)]. Taking average of the IE values determined by VUV-PFI-PE, N2P-PFI-PE, and Stark field extrapolation methods, we obtain a value of 81 285.7±2.8 cm−1 for IE[CS2+(X˜ 2Π3/2)]. For IE[CS2+(2Π1/2)], we recommend a value of 81 727.1±0.5 cm−1 determined by the Rydberg series analysis. A theoretical simulation of the 2Π3/2(000) and 2Π1/2(000) VUV-PFI-PE band profiles reproduces the observed branching ratio of 1.9±0.3 for CS2+(X˜ 2Π3/2)/CS2+(2Π1/2). The relative intensities of vibronic structures observed in the VUV PFI-PE and TPE spectra are in agreement. Evidence is found, indicating that the strongly (Stark field induced) autoionizing Rydberg state, 17pσu, which is (approximate)10 cm−1 below the IE of CS2, has a minor contribution to the observed profile for the X˜ 2Π3/2(000) PFI-PE band. © 1997 American Institute of Physics.

Notes: The photodissociation of dimethylsulfoxide [(CH3)2SO] at 193.3 nm has been investigated using the molecular beam time-of-flight (TOF) mass spectrometric technique. In addition to CH3 and SO, CH3SO is also observed as a stable primary product, indicating that CH3SO+CH3 is an important product channel for the 193.3 nm photodissociation of (CH3)2SO. The analysis of the TOF data provides evidence that SO is formed via a stepwise mechanism: (CH3)2SO+hν (193.3 nm)→CH3SO+CH3→2CH3+SO. The analysis also indicates that (approximate)53% of the primary CH3SO radicals undergo further dissociation to produce CH3+SO, yielding a quantum yield of (approximate)1.53 for CH3. Within the sensitivity of our experiment, the product channel of CH3SCH3+O is not found. The angular distribution for the formation of CH3SO+CH3 is found to be isotropic, an observation consistent with a predissociation mechanism, in which the dissociation of photoexcited (CH3)2SO is slow compared to its rotational period. The energetics for selected dissociation reactions of (CH3)2SO have also been investigated by ab initio calculations at the G2(MP2) level of theory. The experimental dissociation energy at 0 K (53±2 kcal/mol) for the CH3–SOCH3 bond obtained here is in excellent agreement with the theoretical prediction of 52.6 kcal/mol. © 1997 American Institute of Physics.

Notes: The vacuum ultraviolet (VUV) pulsed field ionization photoelectron (PFI-PE) spectra for CH3SH and CH3CH2SH have been obtained near their ionization thresholds. Using a semiempirical simulation scheme, we have obtained satisfactory fits to fine structures resolved in the VUV-PFI-PE spectra, yielding accurate ionization energies of 76 256.3±2.9 cm−1 (9.454 58±0.000 36 eV) and 74 948.7±2.9 cm−1 (9.292 46±0.000 36 eV) for CH3SH and CH3CH2SH, respectively. © 1998 American Institute of Physics.

Notes: The photodissociation of acetophenone (C6H5COCH3) at 193 and 248 nm has been studied using the time-of-flight mass spectrometric technique. For hν=193 nm, two major primary channels, C6H5COCH3+hν→C6H5CO+CH3 [channel (1)] and C6H5+CH3CO [channel (2)], are observed with comparable cross sections. Data analysis shows that (approximate)30%–50% of primary C6H5CO and CH3CO radicals further decomposes, yielding secondary products C6H5+CO and CH3+CO, respectively. The translational energy release measurements indicate that for both channels (1) and (2) at 193 nm, (approximate)25%–30% of the available energy is channeled into kinetic energies of the primary photofragments. Measurements at hν=248 nm reveal that the branching ratio of channel (2) to channel (1) is (approximate)0.01. For channel (1) at hν=248 nm, (approximate)42% of the available energy is directed as the kinetic energy of the photofragments. The observed maximum kinetic energy release for channel (1) at 248 nm yields a value of 85.0±2.2 kcal/mol for the C6H5CO–CH3 bond dissociation energy at 0 K (D0). The photofragment angular distributions are found to be isotropic for both channels (1) and (2) at hν=193 nm and for channel (1) at hν=248 nm. A minor photodissociation channel C6H5COCH3+hν→C6H5CH3+CO is identified at both hν=193 and 248 nm. The energetics for the dissociation reactions of acetophenone have also been investigated using ab initio Gaussian-2-type procedures. The heats of formation at 0 K (ΔfH°0) for C6H5CO and C6H5 calculated using the isodesmic reaction scheme are 33.9±1.3 and 87.6±1.0 kcal/mol, respectively. These results suggest that the literature ΔfH°0 values for C6H5CO and C6H5 are likely to be low by 3–4 kcal/mol. These theoretical ΔfH° values for C6H5CO and C6H5 yield a theoretical D0(C6H5CO–CH3) value of 85.1±1.4 kcal/mol, which is in excellent accord with the experimental results obtained in the present study. © 1997 American Institute of Physics.

Notes: The kinetic energy release spectra for SH resulting from the 193 nm laser photofragmentation of HSCH2CH2SH have been measured. On the basis of the observed maximum kinetic energy for the formation of HS+CH2CH2SH, a value of 74±2 kcal/mol is derived for the bond dissociation energy of HS–CH2CH2SH at 0 K [D0(HS–CH2CH2SH)]. Angular distribution measurements for SH yield an anisotropic parameter β=−0.4±0.1 for the HS+CH2CH2SH channel, indicating that the C–S bond fission is fast with respect to molecular rotation. The energetics for the formation of HS+CH2CH2SH from HSCH2CH2SH have been investigated using the Gaussian-2 (G2) and G2(MP3) ab initio quantum chemical procedures. The G2/G2(MP3) calculations give a prediction of 72.5 kcal/mol for D0(HS–CH2CH2SH), in excellent agreement with the experimental value. Ab initio first-order configuration interaction calculations have also been made to examine the possible excited state of HSCH2CH2SH involved in the photodissociation process and to rationalize the observed angular distribution for the HS+CH2CH2SH channel. © 1996 American Institute of Physics.