Richard P. Tuckett

Fragmentation of valence electronic states of CF+4 and SF+6 studied by threshold photoelectron-photoion coincidence spectroscopy

Abstract Threshold photoelectron-photoion coincidence (TPEPICO) spectroscopy has been used to measure, state selectively, the decay pathways of all the valence states of CF+4 and SF+6 in the range 15–28 eV. Radiation in the vacuum UV from a synchrotron radiation source ionises the parent molecule, and the electrons and ions are detected by threshold electron analysis and time-of-flight mass spectroscopy, respectively. TPEPICO spectra are recorded continuously as a function of photon energy, allowing both threshold photoelectron spectra and yields of all the fragment ions to be obtained. Kinetic energy releases are measured at fixed photon energies with good time resolution. The results orf the X 2T1, A 2T2 and B 2E states of CF+4 (all below the energy of He I radiation) confirm earlier measurements; results for the C 2T2 and D 2A1 states at 21.7 and 25.1 eV, respectively, are new. They confirm previous measurements that radiative decay from both states is an important process, and we have measured their state-selected fluorescence quantum yields. For SF+6, fluorescence does not occur from any of the excited valence states, and we have measured the fragmentation channels and branching ratios for all of the valence states.

Decay pathways of excited electronic states of Group IV tetrafluoro and tetrachloro molecular ions studied with synchrotron radiation

This paper describes experiments to probe the dynamics and decay pathways of the C 2T2 and D 2A1 excited electronic states of Group IV tetrahalide molecular ions MX+4 (M=C, Si, Ge; X=F, Cl) in the gas phase. Tunable vacuum UV radiation from a synchrotron source is used to ionize MX4 into these electronic states of MX+4. Flouorescence from ions initially produced or from fragments is monitored undispersed by a suitable pm tube. When the synchrotron is operated cw, such synchrotron‐induced fluorescence spectra give energy thresholds for fluorescence, and in favorable cases an estimate of the fluorescence quantum yield (of MX+4 C or D) can be made. When the synchrotron is operated pulsed, radiative lifetimes can be measured as a function of excitation energy. The fluorides and chlorides display very different decay properties. The dynamical behavior of these states is rationalized (a) with respect to their spectroscopic properties, and (b) with respect to the dissociation channels energetically ‘‘open’’ ...

Nonradiative decay pathways of electronic states of group IV tetrafluoro and tetrachloro molecular ions studied with synchrotron radiation

The nonradiative decay channels of the valence electronic states of the gas‐phase tetrahedral ions CF+4, SiF+4, CCl+4, SiCl+4, and GeCl+4 have been studied in the range 35–100 nm by a novel form of photoionization mass spectrometry. Tunable vacuum UV radiation from a synchrotron source ionizes the parent neutral molecule, and electrons and ions are detected by the photoelectron–photoion coincidence technique. The experiment is repeated continuously as a function of photon energy, and a three‐dimensional histogram of photon energy versus ion time of flight versus coincidence count rate is produced. By taking cuts through this histogram, photoionization curves for the different fragment ions can be extracted. The appearance energies of the fragment ions (e.g., CF+2 from CF4, CCl+ from CCl4) occur at the adiabatic ionization potential of an electronic state of the parent ion, and not at the thermodynamic appearance energy of that ion. Attempts to measure the kinetic‐energy releases in the fragmentation pathw...

Use of threshold electron and fluorescence coincidence techniques to probe the decay dynamics of the valence states of CF+4, SiF+4, SiCl+4, and GeCl+4

Threshold photoelectron–photoion coincidence (TPEPICO), photoion–fluorescence coincidence (PIFCO), and threshold photoelectron–fluorescence coincidence (TPEFCO) spectroscopies have been used to measure, state selectively, the decay pathways of all the valence states of four gas‐phase tetrahedral ion CF+4, SiF+4, SiCl+4, and GeCl+4 in the range 11–26 eV. Vacuum UV radiation from a synchrotron source dispersed by a 5 m normal‐incidence McPherson monochromator ionizes the parent molecule, and electrons and ions are detected by threshold electron analysis and time‐of‐flight mass spectrometry, respectively. Undispersed fluorescence from the interaction region can also be detected, allowing the three different types of coincidence experiment to be performed. The optimum resolution of the monochromator is matched to that of the threshold analyzer, and this work improves on preliminary results using a 1 m Seya monochromator [Chem. Phys. 174, 441 and 453 (1993)] where the resolution of the spectra was limited by t...

State-specific rate constants for the relaxation of O2(X 3∑g−) from vibrational levels v=8 to 11 by collisions with NO2 and O2

Abstract Rate constants are reported for the state-specific vibrational relaxation of O 2 (8⩽ v ⩽11) by NO 2 and O 2 at 295 K. O 2 is formed in vibrational levels up to v =11 by the reaction of O( 3 P) atoms with NO 2 . The oxygen atoms were created by partial photolysis of NO 2 at 355 nm using a frequency-tripled Nd:YAG laser, and the kinetics of O 2 in particular v levels observed by laser-induced fluorescence in the (0, v ) bands of the B 3 ∑ u − -X 3 ∑ g − system. The rate constants for relaxation of O 2 by NO 2 in units of 10 −13 cm 3 molecule −1 s −1 , are 5.5±1.2, 6.8±1.1, 9.3± 1.2, 7.0±0.8 for v =8, 9, 10, 11, respectively, and by O 2 , in units of 10 −14 cm 3 molecule −1 s −1 , are 8.5±1.7, 10.8±1.7, 8.2±1.4, 7.5±0.3 for v =8, 9, 10, 11, respectively. The results are compared with those obtained previously for higher vibrational levels of O 2 , and their implications for atmospheric chemistry are discussed briefly.

The vacuum-UV absorption spectrum of SF5CF3; implications for its lifetime in the earth’s atmosphere

Abstract Using vacuum-UV radiation from a synchrotron source, the absorption spectrum of SF5CF3 has been measured in the range 50–150 nm at a resolution of 0.12 nm. The cross-section at the Lyman-α wavelength of 121.6 nm is 1.5±0.3×10 −17 cm 2 molecule −1 . The loss of SF5CF3 on a molecular basis from the earth’s atmosphere is dominated, not by photon-induced dissociation, but by electron attachment in the mesosphere above 60 km, yielding SF5−. The lifetime of SF5CF3 in the earth’s atmosphere, however, is determined primarily by the meterological conditions that transport it from the earth’s surface to the mesosphere, and not by processes that occur in that region. By comparison with data for SF6, a lifetime of ca. 1000 years for SF5CF3 is estimated.

Electronic emission spectroscopy of Group IV tetrachloro molecular ions

Two broad continuous bands are observed in the visible region following electron impact ionization of a He‐seeded molecular beam of SiCl4 or GeCl4. By using tunable vacuum UV radiation from a synchrotron source to measure the threshold energy at which the fluorescence bands occur, it is shown that the bands are related to the initial formation of the C 2T2 state of the parent ion SiCl+4/GeCl+4. By comparison with photoelectron data, the bands are assigned to bound–free transitions in SiCl+4/GeCl+4 C 2T2–A 2T2 and X 2T1. The C state of CCl+4 does not fluoresce. The results are compared with the electronic emission spectra of the three tetrafluoro molecular ions.

Fragmentation of the valence states of CF2Cl2+, CF2H2+, and CF2Br2+ studied by threshold photoelectron–photoion coincidence spectroscopy

Using tunable vacuum–ultraviolet radiation from a synchrotron, the decay pathways of the valence electronic states of CF2X2+ (X=Cl, H, Br) in the range 10–25 eV have been determined by threshold photoelectron–photoion coincidence spectroscopy. The ions are separated by a linear time-of-flight mass spectrometer. Coincidence spectra are recorded continuously as a function of energy, allowing threshold photoelectron spectra and yields of the fragment ions to be obtained. At fixed photon energies, spectra are recorded with improved time resolution, allowing the mean total translational kinetic energy, 〈KE〉t, into some dissociation channels to be determined. By comparing the 〈KE〉t values for single-bond fragmentations with those predicted for the limiting extremes of a statistical and an impulsive dissociation, information on the nature of the photodissociation dynamics can be inferred. The excited states of all three parent cations show some evidence for isolated-state behavior. With CF2Cl2+ and CF2H2+, this ...

The vibrational state distributions in both products of the reaction: O(3P) + NO2 → O2 + NO

Abstract A laser pulse-and-probe method has been used to determine the nascent vibrational populations in NO( v =0–4) and O 2 ( v =6–11) formed in the thermal reaction: O( 3 P) + NO 2 → O 2 ( v ) + NO( v ). A frequency-tripled Nd: YAG laser is used to photolyse NO 2 , diluted tenfold in Ar, and laser-induced fluorescence spectroscopy in the NO A 2 Σ + -X 2 Π and O 2 B 3 Σ − u -X 3 Σ − g electronic band system is used both to follow the kinetics of individual vibrational states and to determine the nascent vibrational distributions. The majority of the NO product is formed in v = 0 and the average vibrational yield is ≈ 4.6%. The O 2 populations fall monotonically from v = 6 to 11 in a distribution close to what is expected on prior grounds. Based on a surprisal analysis, the average vibrational energy yield in O 2 is ≈ 26%. The nature of the reaction dynamics is discussed.

State-specific rate constants for the relaxation of O2(X 3Σg–, ν= 8–11) in collisions with O2, N2, NO2, CO2, N2O, CH4 and He

Rate constants have been measured at 295 K for the state-specific vibrational relaxation of O2(8 ⩽ν⩽ 11) by several gases, M = NO2, O2, CO2, N2O, CH4 and He. Upper limits are reported for the rate constants for relaxation by N2. Vibrationally excited O2 was formed in levels up to ν= 11 by the reaction of O(3P) atoms with NO2. The O(3P) atoms were created by partial photolysis of NO2 at 355 nm using a frequency-tripled Nd: YAG laser, and the kinetics of O2 in individual ν levels observed by laser-induced fluorescence in the (0, ν) bands of the B 3Σu––X3Σg– system. Except with He as collision partner, it appears that relaxation occurs by single quantum, intermolecular, vibration–vibration (V–V) energy exchange. Comparison of the rate constants for relaxation by different collision partners demonstrates again the importance of near-resonance and long-range forces in facilitating V–V exchange, with IR active modes accepting vibrational quanta more readily than IR inactive modes. The results are compared with those obtained previously for higher vibrational levels of O2, with estimates of the relaxation rates estimated according to a version of Sharma–Brau theory, and the implications of the results for atmospheric chemistry are discussed briefly.