Ian R. Lambert

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...

Translational spectroscopy of H(D) atom fragments arising from the photodissociation of H2S(D2S): a redetermination of D00(S–H)

The technique of H/D atom photofragment translational spectroscopy has been used to further investigate the collision-free photodissociation of H2S and D2S molecules both in the near ultraviolet (at 218.2 and 221.6 nm) and in the vacuum ultraviolet (at 121.6 nm). Measurements of the H/D atom photofragment angular distributions confirms that the near UV dissociation occurs promptly, following a perpendicular photo-excitation. More than 99% of the resulting SH/SD fragments are formed in their ground vibronic level, with a ca. 3 : 2 preference in favour of the lower (2Π3/2) spin–orbit component. Product rotation accounts for ca. 1% of the available energy in the case of H2S photolysis at these near UV wavelengths (ca. 2% in the case of D2S dissociation). The groundstate SH/SD photofragments can also be photolysed at these near UV excitation wavelengths. Simulations of the kinetic energy distribution of the resulting H/D atomic fragments show that the secondary photolysis also involves a perpendicular transition, and that the partner S atoms are formed in all three 3PJ spin–orbit states. The product energy disposal following 121.6 nm photolysis of D2S closely parallels that deduced in an earlier study of H2S photodissociation at this same wavelength (Schnieder et al., J. Chem. Phys., 1990, 92, 7027). The D-atom kinetic energy spectrum shows clear evidence for the formation of rovibrationally excited SD(A 2Σ+) fragments amongst the primary products, and also suggests an important role for the three-body dissociation process leading to D + D + S(1D) atoms.Given D00(HS–H)= 31 480 ± 40 cm–1, the present results provide a refined value for the S–D bond strength in the D2S molecule; D00(DS–D)= 32 030 ± 50 cm–1; for the SH and SD radical bond dissociation energies; D00(S–H)= 29 300 ± 100 cm–1 and D00(S–D)= 29 700 ± 100 cm–1, and an improved expression for the potential-energy function for the A 2Σ+ state of the mercapto radical.

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.

Vacuum UV spectroscopy and dynamics of CHF3 : analysis of emissions and evidence for parent ion emission

The fluorescence observed following electron impact and vacuum UV photon excitation of fluoroform (CHF3) are studied using three techniques: (a) electron beam excitation of a supersonic molecular beam with dispersed fluorescence detection; (b) He* and Ne* rare-gas metastable excitation at room temperature with dispersed fluorescence detection; (c) photon excitation at room temperature using tunable vacuum UV radiation from a synchrotron source with undispersed fluorescence detection. Using the first two techniques, the spectra are dominated by CF2 A-[Xtilde] fluorescence. The spectra are continuous owing to the high density of rovibrational transitions, but a vibrational progression in the bending mode is observed in the rare-gas metastable experiment at high gas pressure. Using photons in the energy range 11–35 eV, three distinct fluorescence decay channels are observed: (1) CF3 fluorescence for photon energies 11–13 eV; (2) CF2 fluorescence for energies 14–18 eV; (3) CHF+ 3 fluorescence for energies gre...

Vacuum UV fluorescence excitation spectroscopy of BF3 in the range 45-125 nm : observation of new electronic transitions in the BF2 free radical

The fluorescence following excitation of BF3 is studied using two techniques: (a) He* (23S) metastable excitation with dispersed fluorescence detection, (b) photon excitation using tunable vacuum UV radiation from a synchrotron source with undispersed detection. The He* experiment gives an extensive spectrum between 200 and 300 nm with two long progressions, each of separation 525 ± 30 cm-1. They are assigned to transitions to the v 2 bending mode of BF2 [Xtilde] 2A1, probably from the first excited state A 2B1. Using photons in the energy range 10–28 eV two different fluorescence decay channels are observed: (1) BF2 fluorescence for photon energies below 17eV, (2) BF3 + fluorescence for energies > 21·5eV. The shapes of the excitation functions confirm that (1) is a resonant process via Rydberg states of BF3, whereas (2) is a non-resonant photoionization process. The emitting state in BF3 + is the [Etilde]2A′1 state with a vertical ionization potential of 21·5eV. The two strongest resonant peaks at 13·1 a...

Vacuum UV fluorescence excitation spectroscopy of CF4 and CF3Br in the range 45–140 nm

Abstract Fluorescence excitation spectra of CF 4 and CF 3 Br have been measured in the range 45–95 nm and 50–140 nm respectively using synchrotron radiation. Both resonant and non-resonant peaks are observed, corresponding to neutral fragment (e.g. CF 3 ) and parent ion (e.g. CF + 4 ) emission. Analysis of the fluorescence has shown that bands from CF 4 are due to CF* 3 , CF 2 A 1 B 1 and CF + 4 C 2 T 2 . Bands from CF 3 Br are due to CF* 3 , CF 2 A 1 B 1 and possibly CF 3 Br + Ẽ 2 A 1 . The radiative lifetimes of all the fluorescing bands have been measured. Some of the CF 3 Br bands show bi-exponential behaviour. These results are interpreted by comparison with previous photoabsorption and electron energy loss spectra.

Fragmentation of valence electronic states of SF+6 studied with synchrotron radiation

Fragmentation of the seven valence electronic states of SF+6 has been studied in the range 440–810 A by photoionisation mass spectrometry. The ion yield for SF+5 shows two thresholds at the adiabatic ionisation energy of the ground (X 2T1g) and first two excited states (A2T1u and B 2T2u) of SF+6. SF+4 shows a threshold at the third state (C 2Eg) of SF+6, SF+3 shows two thresholds at the fourth and fith states (D 2T2g and E 2T1u), and SF+2 turns on at the sixth state (F 2A1g). Thus in all cases the appearance energy of a fragment ion occurs at the adiabatic ionisation potential of an electronic state of SF+6, and not at the lower-lying thermochemical energy of the fragment ion. Kinetic energy releases have been measured in SF+5, SF+4 and SF+3 at photon energies just above threshold for production of each ion. In the case of SF+5 a substantial fraction of the available energy is converted into translational energy of the fragments. The relative photoionisation yields of SF+5, SF+4 and SF+3 are compared with values expected from the photoelectron branching ratios of electronic states of SF+6. The latter results overestimate the yields of SF+3 and SF+4 relative to SF+5, and possible explanations are discussed.

Vacuum UV spectroscopy and dynamics of CF3Cl, CF2Cl2 and CFCl3: analysis of emissions and evidence for parent ion emission from CF3Cl+

Fluorescence processes following electron impact, He* and Ne* rare-gas metastable impact, and tunable vacuum UV photon excitation of CF3Cl, CF2Cl2 and CFCl3 are observed. Using the first two techniques, emission spectra are dominated by CF2 A-[Xtilde] fluorescence. Using photons in the energy range 11–35eV, four different decay channels are observed: (1) with CF3Cl, CF3 fluorescence for photon energies 11–13eV; (2) with CF3Cl and CF2Cl2, CF2 fluorescence for energies 13–18 eV, (3) with CF2Cl2, CF fluorescence for energies 18–22 eV; (4) with CF3Cl, parent ion emission for energies greater than 20 eV. No fluorescence of comparable intensity is observed from CFCl3 with photon excitation in this range. The emitting electronic state in CF3Cl+ is the [Etilde]2A1 state with a vertical ionisation potential of 20·2 eV and a radiative lifetime of 25 ns.

Photoionisation mass-spectrometric study of fragmentation of SiBr4 and GeBr4 in the range 400–1220 Å

The non-radiative decay channels of the valence electronic states of SiBr+4 and GeBr+4 have been studied in the range 1220–400 A(10–31 eV) by photoionisation mass spectrometry. Ion-yield curves for the parent ions and for MBr+3, MBr+2, MBr+, M+ and Br+(M = Si, Ge) have been obtained, as well as the relative photoionisation branching ratios. The appearance thresholds for SiBr+3 and GeBr+3 occur at 11.31 and 10.97 eV, respectively. They lie within the Franck–Condon region of the ground state of SiBr+4 and GeBr+4, and are at the thermodynamic thresholds for SiBr+3+ Br and GeBr+3+ Br. The smaller fragment ions have appearance thresholds which relate to energies of excited electronic states of SiBr+4, and GeBr+4, and not to the lower-lying thermodynamic energy of the fragment ion. The results are discussed with reference to our earlier work on radiative decay from excited states of SiBr+4 and GeBr+4(J. Chem. Soc., Faraday Trans., 1990, 86, 2021). We have obtained a new value for the ionisation potential of SiBr3 of 7.6 ± 0.4 eV, and we suggest that the previously accepted value for SiBr2(12 ± 1 eV) is ca. 3.5 eV too high.