K. Norwood

A photoion–photoelectron coincidence study of Arn (n=2–4)

Photoion–photoelectron coincidence (PIPECO) spectra for Ar+n (n=2–4) in the wavelength region of 750–875 A have been measured at different nozzle stagnation pressures. The ionization energies (IE) for the ground Ar+2[I(1/2)u] and Ar+3[I(1/2)u] states are determined to be 14.518±0.017 eV (854±1 A) and 14.350±0.033 eV (864±2 A), in agreement with the findings of previous photoionization experiments. The IE for Ar+2[II(1/2)u] is found to be ≲15.83 eV (783 A). The observation of the Ar+2[II(1/2)u] PIPECO band supports the interpretation that Ar+2[II(1/2)u] is metastable with a lifetime longer than 47 μs, a value in accord with the calculated radiative lifetime of 90.9 μs for the II(1/2)u →I(1/2)g transition. The PIPECO spectrum for Ar+2 is dominated by autoionization structure similar to that resolved in the photoionization efficiency spectrum for Ar+2, indicating that a significant fraction of electrons produced by these autoionizing states are slow electrons with near‐zero kinetic energies. Evidence is foun...

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 state‐selected study of the unimolecular decomposition of C2H+2(Ã,B̃) using the photoion–photoelectron coincidence method

The photoion–photoelectron coincidence spectra for C2H+ and C2H+2 have been measured in the wavelength range of 645–765 A. The C2H+2(A 2Ag,B 2∑+u) ions prepared with internal energies above 17.39 eV are found to dissociate completely into C2H++H in the temporal range <12 μs. An upper bound of 17.33±0.05 eV is determined for the appearance energy of the process C2H2+hν→C2H++H+e− at 0 K.

A Study of intramolecular charge transfer in mixed Ar/CO dimer and trimer ions using the photoion-photoelectron coincidence method

Abstract The photoion-photoelectron coincidence (PIPECO) spectra for ArCO + in the wavelength region of 620–940 A have been measured at different nozzle expansion conditions. The ionization energy (IE) for CO + (≈X)·Ar is determined to be 13.33±0.06 eV (930±4 A). Using this value, the IE for CO + (≈X), and the estimated binding energy for Ar·CO, we calculate a value of 0.70 ± 0.06 eV for the dissociation energy of CO + (X)·Ar. The excited Ar + ( 2 P J )·CO and CO + (≈A or ≈B)·Ar dimer ions formed by the photoionization of ArCO are found to be dissociative. The dissociation of Ar + ( 2 P J )·CO is rationalized by a stepwise mechanism involving the formation of a vibrationally excited CO + (≈X,ν′)·Ar complex by near-resonance intramolecular charge transfer and the subsequent dissociation of the complex by vibrational predissociation. Assuming that the radiative lifetimes of CO + (≈A,≈B) and CO + (≈A,≈B)·Ar are identical, we estimate that the dissociation lifetimes of·ar CO + (≈A)·Ar and CO + (≈B)·Ar are shorter than the radiative lifetimes of CO + (≈A) (≈4 μs) and CO + (≈B) (≈50 ns), respectively. The formation of ArCO + and (CO) 2 + from the fragmentation of excited Ar + ( 2 P J )·(CO) 2 and Ar + ( 2 P J )·(ArCO) trimer ions is efficient. The good agreement found in the comparison of the measured vibrational distribution of CO + (≈X,ν′) produced in the Ar + ( 2 P 3 2 )+CO charge transfer reaction and the profile for the ArCO + and (CO) 2 + coincidence electronic bands arisen from the dissociation of Ar + ( 2 P 3 2 )·(CO) 2 and/ or Ar + ( 2 P 3 2 )·(ArCO) implies the involvement of near-resonance intramolecular charge transfer between Ar + ( 2 P 3 2 ) and a CO molecule in the trimer ions prior to their decomposition. Evidence supporting a binding energy for Ar + ( 2 P 3 2 )·CO to lie in the range of 0.66–0.97 eV is found.

A photoelectron–photoion coincidence study of H2O, D2O, and (H2O)2

Photoelectron–photoion coincidence (PEPICO) data for OH+(OD+), H+(D+), and H2O+ (D2O+) from H2O (D2O) have been obtained in the region of 625–700 A. The PEPICO measurements allow the construction of breakdown diagrams for the unimolecular dissociation of energy‐selected H2O+ and D2O+ in the B 2B2 state. The breakdown diagrams for H2O+(B 2B2) and D2O+(B 2B2) in the internal energy range of 129–166 kcal/mol are essentially identical. The branching ratios observed for H+ (D+) are higher than those reported previously. About 3%–5% of stable H2O+ (D2O+) is observed in the time scale of ≊10 μs. These stable H2O+ (D2O+) ions are attributed to ultrafast B 2B2→A2A1 nonradiative relaxation followed by the radiative stabilization from H2O+(A2A1) [D2O+(A2A1)] to H2O+ (X2B1) [D2O+(X2B1)]. This observation also supports that the formation of H+ (D+) via the H2O+ (A 2A1)[D2O+(A2A1)] state is a viable process. The relative state‐selected cross sections for the reaction H2O+ (X2B1,A2A1; ν1, ν2)+H2O→H3O+ +H at...

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 photoion–photoelectron coincidence study of Kr and Xe dimers

The photoion–photoelectron coincidence (PIPECO) spectra for Kr+2 and Xe+2 in the wavelength regions of 825–970 and 900–1030 A, respectively, have been measured at different nozzle temperatures and stagnation pressures (P0). The ionization energies (IE) for Kr2 and Xe2 to Kr+2[I(1/2)u] and Xe+2[I(1/2)u] determined by the PIPECO spectra are in excellent agreement with the results of previous photoionization experiments. The PIPECO measurements for Kr+2 and Xe+2 also provide lower limits for the IEs of Kr2 and Xe2 to Kr+2[II(1/2)u] and Xe+2[II(1/2)u]. The PIPECO spectra for Kr+2 and Xe+2 display strong autoionization structures similar to those resolved in the corresponding photoionization efficiency spectra, indicating that a significant fraction of autoionizing electrons are slow electrons with near zero kinetic energies. The extreme weakness of the Kr+2[II(1/2)u] andXe+2[II(1/2)u] PIPECO bands observed at low P0 support the conclusion that excited Kr+2[II(1/2)u] and Xe+2[II(1/2)u] ions are dissociative wi...

A photoion–photoelectron coincidence study of (CO)2 and (CO)3

The photoion–photoelectron coincidence (PIPECO) spectra for (N2)+2 in the wavelength range 650–866 A have been measured at different nozzle stagnation pressures. The formation of stable (N2)+2 from fragmentation of excited (N2)+n cluster ions initially produced by photoionization of (N2)n, n≥3, is efficient. For nozzle expansion conditions which minimize the production of (N2)n, n≥3, the intensities for the N+2(A,B)⋅N2 PIPECO bands are found to be negligibly small compared to that of the N+2(X)⋅N2 PIPECO band, indicating that the electronically excited N+2(A,B)⋅N2 dimer ions are dissociative in temporal ranges <42 μs. Assuming that the radiative lifetimes for N+2(A,B) and N+2(A,B)⋅N2 are identical, we estimate that the dissociative lifetimes for N+2(A)⋅N2 and N+2(B)⋅N2 are ≲10 μs and ≲60 ns, respectively. The ionization energy for (N2)2 is determined to be 14.50±0.08 eV (855±5 A), suggesting that N+2(X)⋅N2 is bound by 1.09±0.08 eV. The PIPECO data for (N2)+2 presented here and those for (CO)+2...

A Photoion-Photoelectron Coincidence Study of (CO)2

Molecular beam photoionization1 and equilibrium2 mass spectrometric measurements provide valuable energetic information about dimer and cluster ions in their ground state. In spite of recent intense research activities in cluster ion chemistry, little is known about the interaction energies of an excited state ion with neutral molecules. A systematic method for determining the binding energies of a ground state as well as an excited state ion with neutral species is to measure the adiabatic ionization energies (IE) of the appropriate clusters by photoelectron spectroscopic techniques. The concentrations of clusters produced in a supersonic beam are usually much lower than that of the monomers. This, together with the fact that photoelectron bands of monomers and clusters often overlap in energy, makes the measurement of the photoelectron spectrum (PES) of a specific cluster difficult.