Kenneth J. Boyle

Vacuum-UV fluorescence spectroscopy of CF3X(X=F,H,Cl,Br) in the range 10–30 eV

Abstract The vacuum-UV and visible spectroscopy of CF3X(X=F,H,Cl,Br) using fluorescence excitation and dispersed emission techniques is reported. Fluorescence excitation spectra have been recorded following excitation using dispersed synchroton radiation in the energy range 10–30 eV at an average resolution of ∼0.06 eV. By comparison with vacuum-UV absorption and electron energy loss spectra, assignments have been made for all the peaks in the Rydberg spectra of the titled molecules that photodissociate to a fluorescing state of a fragment. Dispersed emission spectra have been recorded at the energies of all the peaks in the excitation spectra. Three distinct decay channels are observed: (a) CF3 fluorescence for photon energies in the range 10.5 to 13.5 eV, (b) CF2 fluorescence for photon energies in the range 14.7 to 15.7 eV, (c) for photon energies in the range 20–23 eV parent ion fluorescence in the case of CF4 and CF3H, predominantly CF emission in the case of CF3Cl and CF3Br. These assignments are confirmed by action spectra in which the energy of the vacuum-UV radiation is tuned for detection at a specific (dispersed) wavelength. These results confirm those reported in the previous paper from fluorescence coincidence experiments that parent-ion fluorescence is the major component of the emission for CF4 and CF3H excited in this latter energy range of 20–23 eV, whereas it represents only a minor component for CF3Cl and a negligibly-small component for CF3Br.

Vacuum-UV fluorescence spectroscopy of SiF4 in the range 10–30 eV

The vacuum-UV and visible spectroscopy of SiF4 using fluorescence excitation and dispersed emission techniques is reported. The fluorescence excitation spectrum has been recorded following excitation with synchrotron radiation from the BESSY 1, Berlin source in the energy range 10–30 eV with an average resolution of ∼0.05 eV. By comparison with vacuum-UV absorption and electron energy loss spectra, all the peaks in the Rydberg spectra that photodissociate to a fluorescing state of a fragment have been assigned. Dispersed emission spectra have been recorded at the energies of all the peaks in the excitation spectra. Four different decay channels are observed: (a) SiF3 fluorescence in the range 380–650 nm for photon energies around 13.0 eV, (b) SiF2 a 3B1–X 1A1 phosphorescence in the range 360–440 nm for photon energies in the range 15.2–18.0 eV, (c) SiF2 A 1B1–X 1A1 fluorescence in the range 210–270 nm for photon energies in the range 17.0–20.0 eV, and (d) emission from the D 2A1 state of SiF4+ predomin...

Threshold photoelectron spectroscopy of PX3 (X=Cl, Br), and fragmentation of the valence electronic states of PX3+ studied by TPEPICO spectroscopy

The valence threshold photoelectron spectra (TPES) of phosphorous trichloride (PCl3) and phosphorous tribromide (PBr3) are presented for the first time. Threshold photoelectron–photoion coincidence (TPEPICO) spectroscopy has also been used to measure, state selectively, the decay pathways of the valence states of PX3+ (X=Cl, Br) in the range 10–25 eV. Vacuum-UV radiation from the Daresbury synchrotron source dispersed by a 1 m Seya-Namioka monochromator photoionises the parent molecules, electrons and ions being detected by threshold electron analysis and time-of-flight mass spectrometry, respectively. TPEPICO spectra are recorded continuously as a function of photon energy, allowing coincidence ion yields of the fragment ions to be obtained. The ground and first excited states of both molecular parent ions are stable with respect to dissociation to PX2+, whereas the B2E, C2E, D2A1 and E2E states dissociate exclusively to PX2+. The first experimental observation of the F2A1 state of PBr3+ is presented, which we deduce to have a very small cross-section to photoionisation. Fragmentation of the F2A1 states of both parent ions leads to exclusive production of PX+. Kinetic-energy (KE) releases into PX2++X have been measured at the Franck–Condon maxima of the B2E, C2E, D2A1 and E2E states of PCl3+ and the B2E, C2E and E2E states of PBr3+. For the first time, the method used to calculate the KE releases allows for the full range of daughter ion isotopes to be accommodated.

Vacuum-UV fluorescence spectroscopy of PF3 in the range 9–20 eV

The vacuum-UV and visible spectroscopy of PF3 using fluorescence excitation and dispersed emission techniques is reported. The fluorescence excitation spectrum has been recorded following photoexcitation with monochromatized synchrotron radiation from the Daresbury, UK source in the energy range 9–20 eV with an average resolution of ∼0.015 eV. Transitions to the three lowest-energy bands in the Rydberg spectra show resolved vibrational structure, they are assigned to transitions to the (8a1)−1 4p, 5p, and 6p Rydberg states of PF3, and fluorescence is due to valence transitions in the PF2 radical. From a Franck–Condon analysis of the vibrational structure, it is shown that the FPF bond angle in PF3 increases by ∼14±1° upon photoexcitation. The use of optical filters shows that at least three excited electronic states of PF2 are responsible for the induced emission. Dispersed emission spectra in the UV/visible region have been recorded with an optical resolution of 8 nm at the BESSY 1, Germany synchrotron s...

THE USE OF THRESHOLD PHOTOELECTRON : FLUORESCENCE PHOTON COINCIDENCE SPECTROSCOPY FOR THE MEASUREMENT OF THE RADIATIVE LIFETIMES OF EMITTING STATES OF CF3X+ (X=F, H, CL, BR) IONS

Abstract Two different experimental techniques are used to measure the lifetime of excited electronic states of CF3X+ (X = F, H, Cl, Br) in the gas phase with excitation energies in the range 20–23 eV. Using the single-bunch mode of a synchroton source, the first technique utilises pulsed photon excitation to excite the parent neutral molecule, and the decay of the fluorescence is observed in real time. This technique detects time-resolved fluorescence from all emitters, both neutrals and ions, excited at the proton energy, and is therefore a poor method to measure the lifetime of a parent ion if the ionisation quantum yield at the incident energy is less than unity. Also using tunable vacuum-UV radiation from a synchrotron source but now in the multi-bunch, quasi-continuous mode, the second technique detects coincidences between threshold photoelectrons and fluorescent photons. Because this experiment only detects coincidences in ions and not in neutrals, the decay of the fluorescence in real time is only due to the ionic component of the emission. In this way lifetimes of emitting states of molecular ions, absent of contributions from neutrals, can be measured. With sufficient resolution in both photon source and threshold electron analyser, vibrationally state-selected lifetimes can be measured. We use this coincidence technique to measure the lifetimes of the C2T2 state of CF4+ (v = 6), the D 2A1 state of CF3H+ and the Ẽ2A1 state of CF3Cl+ to be 8.4, 12.6 and 9.4 ns, respectively. The fluorescence quantum yield of the Ẽ2A1 state of CF3Br+ is too small for signal to be observed in the coincidence experiment. These results are compared with those obtained from pulsed photoexcitation of CF3X, and some conclusions are drawn about the nature of the emitters when CF3X is excited at the appropriate energy. In the cases of CF4 and CF3H emission is solely in the parent ion. With CF3Cl parent ion emission is a minor component and the main emitters are excited states of the CF radical. With CF3Br emissions are only due to neutrals with, again, excited states of CF being the main contributor.

Vacuum-UV fluorescence spectroscopy of BF3 and BCl3 in the range 9-22 eV: dispersed spectra and radiative lifetimes

The fluorescence processes following vacuum-UV (VUV) photoexcitation of BF3 and BCl3 in the energy range 9-22 eV have been studied using synchrotron radiation from the BESSY 1, Berlin source. The fluorescence is dispersed in a secondary monochromator, whereby information on the nature of the emitting species is obtained. For excitation energies in the range 13-17 eV, photodissociation of Rydberg states of BF3 produces a substantial branching ratio into two, possibly three, excited valence states of the BF2 free radical, which fluoresce to the ground state. The dispersed spectra are compared with predictions from ab initio calculations of the electronic spectroscopy of this radical. Fluorescence from BCl3 photoexcited in the range 9-22 eV is more complicated, with emissions having already been observed and mostly assigned in BCl2, BCl, BCl3+, BCl2+ and B. Using the single-bunch, pulsed mode of the BESSY 1 source, the radiative lifetimes of emitters from BCl3 lying in the range 3-100 ns have been measured. Our results for BCl3 are compared with data from other techniques. Where more than one emitter is excited at a particular VUV energy, the dispersion of the induced fluorescence through a monochromator means that the lifetimes of the different emitters can be differentiated.

Threshold photoelectron spectroscopy of BCl3 and fragmentation of the valence electronic states of BCl3+, studied by coincidence spectroscopies

The valence threshold photoelectron spectrum (TPES) of BCl3 is reported for the first time. Threshold photoelectron–photoion coincidence (TPEPICO), photoion-fluorescence coincidence (PIFCO) and threshold photoelectron-fluorescence coincidence (TPEFCO) spectroscopies have also been used to measure for the first time, state selectively, the decay pathways of the valence states of BCl3+ in the range 11–19 eV. Vacuum UV radiation from a synchrotron source dispersed by a 5 m normal-incidence McPherson monochromator ionises 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. There are major differences in the intensity distribution of the different bands in the TPES of BCl3 compared with its He I photoelectron spectrum which we attribute to autoionisation. TPEPICO spectra are recorded continuously as a function of photon energy, allowing yields of the fragment ions to be obtained. Over the range 11–19 eV, only the ions BCl2+ and BCl3+ are observed. The ground electronic state of BCl3+ is stable with respect to dissociation to BCl2+, whereas the five excited electronic states fragment to BCl2++ Cl. Kinetic-energy releases into BCl2++ Cl have been measured at the Franck–Condon maxima of the C 2A″2, D 2E′ and E 2A′1 excited states of BCl3+ with good time resolution. PIFCO and TPEFCO spectra are recorded at fixed photon energies. The former experiment can yield the fate of the lower electronic state of the parent ion to which fluorescence occurs. The latter experiment can yield the lifetime of the fluorescing state. Radiative decay is confirmed to occur from the fourth excited state of BCl3+(D 2E′) at 15.5 eV, and the lifetime of this state is measured to be 5.5 ± 0.5 ns. The absence of parent ion signal in the energy-scanning TPEPICO spectrum through the D state allows us to put an upper limit on the fluorescence quantum yield of this state of ca. 0.04.

Vacuum-UV fluorescence spectroscopy of PX3 (X = Cl, Br) in the range 9–25 eV

The vacuum-UV (VUV) and visible spectroscopy of PCl3 and PBr3 using fluorescence excitation and dispersed emission techniques is reported. Non-dispersed fluorescence excitation spectra have been recorded following photoexcitation with monochromatised synchrotron radiation from the Daresbury, UK source in the VUV energy range 9–25 eV, with an average resolution ofca. 0.02 eV. The use of optical filters shows that fluorescence is due to both resonant excitation of Rydberg states of PX3 (X = Cl,Br) which photodissociate to a fluorescing state of a fragment (e.g. PX2, X2) and to non-resonant excitation of excited electronic states of PX3+. Dispersed emission spectra in the UV–VIS have been recorded, with an optical resolution of ca. 4–8 nm, at the BESSY 1, Germany synchrotron source at the energies of the peaks in the excitation spectra. Four different decay channels areobserved. (a) PCl2 2B2–2B1 fluorescence in the range 350–700 nm for photon energies in the range 10–12 eV; the analogousemission in PBr2 falls outside the sensitivity range of the detection systems. (b) Cl2 and Br2 D′ 2 3Πg–A′ 2 3Πu emission at ca. 260 and 290 nm, respectively, for photon energies in the range 13–18 eV. (c) Emission from the Ẽ2E state of PCl3+ and PBr3+ for photon energies in excess of the adiabatic ionisation energies of these states of 14.6 and 13.9 eV, respectively. (d) Atomic emission from excited states of the P atom for photon energies in the range 19–25 eV. Action spectra have also been recorded at BESSY 1, in which the energy of the VUV radiation is scanned with detection of the fluorescence at a fixed, dispersive wavelength. Energy thresholds for production of the emitters can then be determined. The 2B2 state of PCl2 has a threshold energy which lies well above the thermodynamic energy needed to produce PCl2 2B2 + Cl. We conclude that it forms via a one-step P–Cl bond cleavage of a Rydberg state of PCl3. From the threshold energy for X2 D′ 2 3Πg production, we conclude that this excited ion-pair state of X2 forms in combination with the ground electronic state of PX, and not with isolated P + X atoms. Theseproducts form via a single-step photodissociation of a Rydberg state of PX3 which must now pass through a tightly constrainedtransition state. By contrast, excited states of the phosphorus atom are formed by a sequential, multi-step photodissociation of PX3*. Now the thresholds for these emissions correspond to the thermodynamic energies to form the emitting P* atom in combination with three halogen atoms.

Vacuum-UV photofragmentation of polyatomic ions using coincidence techniques

Abstract We describe an apparatus for performing three types of coincidence experiment to study state-selected photofragmentation of polyatomic ions in the gas phase; threshold photoelectron–photoion coincidences (TPEPICO), photoion–fluorescence coincidences (PIFCO), and threshold photoelectron–fluorescence coincidences (TPEFCO). Photoionisation in the vacuum-UV, 10–25 eV, is obtained using tunable radiation from the Daresbury, UK Synchrotron Radiation Source. Examples are taken from published work in CF4+, PCl3+ and PBr3+, and a range of perfluorocarbon cations CxFy+.

Vacuum-UV fluorescence spectroscopy of polyatomic molecules

Abstract Experiments are described in which UV and visible fluorescence is observed following vacuum-UV photoexcitation of a range of polyatomic molecules in the gas phase. Tunable radiation in the energy range 8–25 eV from synchrotron sources at Daresbury, UK and BESSY 1, Germany is used as the photoexcitation source. Non-dispersed fluorescence excitation spectra and dispersed fluorescence spectra are recorded at Daresbury and BESSY 1, respectively. The experiments are sensitive to Rydberg states of molecules that photodissociate to an excited state of a fragment that fluoresces, and to valence states of the parent molecular ion that fluoresces. Using single-bunch mode of BESSY 1, lifetimes of the emitters in the range ca. 3–100 ns are measured. Examples are taken from work either published or in press in SiF 4 and GeF 4 , PF 3 , MCl 4 (M=C, Si, Ge), and CXCl 3 (X=H, F, Br).