S. Nourbakhsh

A 193 nm laser photofragmentation time‐of‐flight mass spectrometric study of CS2 and CS2 clusters

A crossed laser and molecular beam photofragmentation apparatus is described. The apparatus is equipped with a rotatable molecular beam source and a translationally movable ultrahigh vacuum mass spectrometer for time‐of‐flight (TOF) measurements. Using this apparatus we have measured the TOF spectra of S and CS resulting from the photofragmentation processes, CS2+hν(193 nm)→CS(X,v)+S(1D or 3P). The translational energy distributions of photofragments derived from the S and CS TOF spectra are in good agreement. This observation, together with the finding that the TOF spectra of S and CS are independent of laser power in the 25–150 mJ range, shows that the further absorption of a laser photon by CS to form C(3P)+S(3P) within the laser pulse is insignificant. The TOF spectra of S obtained at electron ionization energies of 20 and 50 eV are indiscernible, indicating that the contribution to the TOF spectrum of S from dissociative ionization of CS is negligible at electron impact energies ≤50 eV. The thermoche...

Vacuum ultraviolet photodissociation and photoionization studies of CH3SH and SH

The kinetic energy releases of the photodissociation processes, CH3SH+hν (193 nm)→CH3+SH, CH3S+H, and CH2S+H2, have been measured using the time‐of‐flight mass spectrometric method. These measurements allow the direct determination of the dissociation energies for the CH3–SH and CH3S–H bonds at 0 K as 72.4±1.5 and 90±2 kcal/mol, respectively. The further dissociation of SH according to the process SH+hν (193 nm)→S+H has also been observed. The appearance energy (AE) of S produced in the latter process is consistent with the formation of S(3P)+H. The photoelectron–photoion coincidence (PEPICO) spectra for CH3SH+, CH3S+ (or CH2SH+), and CH2S+ from CH3SH have been measured in the wavelength range of 925–1460 A. The PEPICO measurements make possible the construction of the breakdown diagram for the unimolecular decomposition of internal‐energy‐selected CH3SH+ in the range of 0–83 kcal/mol. The AE measured for CH2S+ is consistent with the conclusion that the activation energy is negligible for 1,2‐H2 eliminati...

Vacuum ultraviolet photodissociation and photoionization studies of CH3SCH3 and CH3S

We have measured the translational energy releases of the laser photodissociation processes CH3SCH3+hν (193 nm)→CH3+CH3S [process (1)] and CH3SCH2+H [process (2)]; and CH3S+hν (193 nm)→S+CH3 [process (3)]. The onsets of the translational energy distributions for photofragments of processes (1) and (2) allow the direct determination of 74.9±1.5 and 91±2.5 kcal/mol for the dissociation energies of the CH3–SCH3 and H–CH2SCH3 bonds at 0 K, respectively. The threshold observed for S formed by process (3) is consistent with the conclusion that the production of S(3P) is small compared to S(1D). The photoelectron–photoion coincidence (PEPICO) spectra for CH3SCH+3, CH3SCH+2, CH3S+ (or CH2SH+ ), and CH2S+ resulting from photoionization of CH3SCH3 have been measured in the wavelength region of 900–1475 A. The PEPICO study allows the construction of a detailed breakdown diagram for the formation of CH3SCH+2, CH3S+ (or CH2SH+ ), and CH2S+ from energy‐selected CH3SCH+3 ions.

A 193 nm laser photofragmentation time‐of‐flight mass spectrometric study of CH3SSCH3, SSCH3, and SCH3

We have measured the time‐of‐flight (TOF) spectra for SCH3, CH3, and SSCH3 formed in the photodissociation processes, CH3SSCH3+hν(193 nm)→2SCH3 and CH3+SSCH3. The dissociation energies for the CH3S–SCH3 and CH3SS–CH3 bonds determined at 0 K by the TOF measurements are 72.4±1.5 and 55.0±1.5 kcal/mol, in agreement with the literature values. The threshold value for the formation of S2 measured by the TOF spectrum for S2 is in accord with the thermochemical threshold for the process, SSCH3+hν(193 nm) →S2+CH3. The threshold energy determined from the TOF spectrum for S is found to be consistent with the thermochemical threshold for the photodissociation process, SCH3+hν(193 nm) →S(1D)+CH3, an observation supporting that S atoms are not produced in the ground S(3P) state in the 193 nm photodissociation of SCH3. This observation is rationalized by symmetry correlation arguments applied between the S+CH3 product and SCH3 states.

Absolute state-selected and state-to-state total cross sections for the reaction Ar+(2P3/2,1/2)+O2

Absolute spin–orbit state‐selected total cross sections for the reactions, Ar+(2P3/2,1/2)+O2→O+2+Ar [reaction (1)], O++O+Ar [reaction (2)], and ArO++O [reaction (3)], have been measured in the center‐of‐mass collision energy (Ec.m.) range of 0.044–133.3 eV. Absolute spin–orbit state transition total cross sections for the Ar+(2P3/2,1/2)+O2 reaction at Ec.m.=2.2–177.6 eV have also been examined. The appearance energies for the formation of O+ (Ec.m.=2.9±0.2 eV) and ArO+ (2.2±0.2 eV) are in agreement with the thermochemical thresholds for reactions (2) and (3), respectively. The cross sections for O+2, O+, and ArO+ depend strongly on Ec.m. and the spin–orbit states of Ar+, suggesting that reactions (1)–(3) are governed predominantly by couplings between electronic potential energy surfaces arising from the interactions of Ar+(2P3/2)+O2, Ar+(2P1/2)+O2, and O+2+Ar.In the Ec.m. range of 6.7–22.2 eV, corresponding to the peak region of the O+ cross section curve, the cross sections for O+ are ≥50% of those for ...

Theoretical and experimental total state-selected and state-to-state cross sections. III, The (Ar+H2)+ system

A detailed three‐dimensional quantum mechanical study of the (Ar+H2)+ system along the energy range 0.4 eV≤Etot≤1.65 eV is presented. The main difference between this new treatment and the previously published one [J. Chem. Phys. 87, 465 (1987)] is the employment of a new version of the reactive infinite‐order sudden approximation (IOSA), which is based on the ordinary inelastic IOSA carried out for an optical potential. In the numerical treatment we include three surfaces (only two were included in the previous treatment), one which correlates with the Ar+H+2 system and two which correlate with the two spin states of Ar+(2Pj); j=3/2,1/2. The results are compared with both trajectory‐surface‐hopping calculations and with experiments. In most cases, very good agreement is obtained.

Experimental and theoretical total state-selected and state-to-state absolute cross sections. II. The Ar sup + ( sup 2 P sub 3/2,1/2 )+H sub 2 reaction

Total state-selected and state-to-state absolute cross sections for the reactions Ar{sup +}({sup 2}{ital P}{sub 3/2,1/2})+H{sub 2}({ital X},{ital v}=0){r arrow}Ar ({sup 1}{ital S}{sub 0})+H{sup +}{sub 2}({ital {tilde X}},{ital v}{prime}) (reaction (1)), ArH{sup +}+H (reaction (2)), and H{sup +}+H+Ar (reaction (3)) have been measured in the center-of-mass collision energy {ital E}{sub c.m.} range of 0.24--19.1 eV. Absolute spin--orbit state transition total cross sections ({sigma}{sub 3/2{r arrow}1/2},{sigma}{sub 1/2{r arrow}3/2}) for the collisions of Ar{sup +}({sup 2}{ital P}{sub 3/2,1/2}) with H{sub 2} at {ital E}{sub c.m.}=1.2--19.1 eV have been obtained.

Absolute state‐selected and state‐to‐state total cross sections for the Ar+(2P3/2,1/2)+CO2 reactions

Absolute spin–orbit state‐selected total cross sections for the reactions, Ar+(2P3/2,1/2)+CO→CO++Ar [reaction (1)], C++O+Ar [reaction (2)], O++C+Ar [reaction (3)], and ArC++O [reaction (4)], have been measured in the center‐of‐mass collision energy (Ec.m.) range of 0.04–123.5 eV. Absolute spin–orbit state transition total cross sections for the Ar+(2P3/2,1/2)+CO reactions at Ec.m. have also been obtained. The appearance energies (AE) for C+(Ec.m.=6.6±0.4 eV) and O+(Ec.m.=8.6±0.4 eV) are in agreement with the thermochemical thresholds for reactions (2) and (3), respectively. The observed AE for reaction (4) yields a lower bound of 0.5 eV for the ArC+ bond dissociation energy. The kinetic energy dependence of the absolute cross sections and the retarding potential analysis of the product ions support that ArC+, C+, and O+ are formed via a charge transfer predissociation mechanism, similar to that proposed to be responsible for the formation of O+ (N+) and ArO+ (ArN+) in the collisions of Ar+(2P3/2,1/2)+O2(N2).

Photoionization study of supersonically cooled CS formed in the excimer photodissociation of CS2

Abstract The photoionization efficiency spectrum of supersonically cooled CS in the region of 1000-1100 A is presented. The CS transient molecules are prepared by 193 nm photodissociation of a pulsed supersonic CS2 free jet. The ionization energy for CS is determined to be 11.318 ± 0.007 eV. The observed PIE spectrum for CS and that for SO from the 193 nm photodissociation of a SO2 free jet support the conclusion that rotational and vibrational excitations of photofragments relax efficiently in the supersonic expansion.

A 193 nm photodissociation time-of-flight mass spectrometric study of C6H5SH and C6H5SCH3

The kinetic energy releases of the photodissociation processes C6H5SH + h-nu (193 nm) --> C6H5 + SH, and C6H5SCH3 + h-nu (193 nm) --> C6H5 + SCH3 and C6H5S + CH3, have been measured by the time-of-flight mass spectrometric method. The available energies for these processes are found to appear predominantly (almost-equal-to 70-80%) as internal excitations for the photofragments. Based on the thresholds of kinetic energy release spectra, we obtain values of 87.4 +/- 2.0, 87 +/- 3, and < 81 +/- 7 kcal/mol for the dissociation energies of the C6H5-SH, C6H5S-CH3, and C6H5S-CH3 bonds at 0 K.