VUV Fluorescence Research Articles

Study of the sulphur-containing molecules in the EUV region. IV. The dissociation processes of SO2in the 760 AA region

The fluoresence throughout the VUV and violet regions has been observed from photoexcitation of SO2, and the absolute fluorescence cross sections of the VUV emissions have been measured at selected atomic emission lines and in the synchrotron radiation continuum from 550 to 760 A. Emissions from the O I 1304 A and several S I lines account for the fluorescence observed in the VUV region, while the fluorescence observed in the UV and violet regions is due to excited SO and, also, possibly to excited SO2(+) and SO(+). Structure in the VUV fluorescence excitation function is attributed to predissociation of Rydberg states. The VUV fluorescence resulting from an O2 elimination process, SO2 + h times nu yielding metastable S + O2, has been observed in the present work although the threshold for this process has not been determined. The threshold corresponding to the total dissociation process, SO2 + h times nu yielding metastable S + 2O, has been measured and found to agree reasonably with the expected value calculated from thermochemical data. The processes for the formation of S(+) are also discussed.

Detection of the a 1Πg (v′=0, 1)←X 1Σ+g (v″=0) transition in N2 by laser-induced fluorescence

The a1Πg(v′=0.1)←X1Σ+g(v″=0) transitions in N2, part of the Lyman–Birge–Hopfield bands, have been seen by two-photon absorption detected by VUV fluorescence. Fluorescence was observed over a pressure range from 5 m Torr to 10 Torr. From the lifetime decrease with pressure at low pressure, quenching rate constants were determined, which are equivalent to cross sections in the range of 2.6×10−16 cm2. At higher pressures a slow decay was observed that is interpreted as being due to the a′ state which acts as an energy reservoir with a pressure dependent decay constant. We anticipate that laser-induced fluorescence will be a useful scheme for measuring ground state rotational and vibrational populations for the nitrogen molecule.

VUV fluorescence following photodissociation of N2O at 193 nm

Photolytic studies performed at 193 nm demonstrate that NO in the highly excited D(v = 1,5) and E(v = 0) states is generated from N2O during irradiation in three sequential steps involving photodissociation, chemical reaction, and photoexcitation. The resulting NO fluorescence (160–230 nm) was analyzed with a system of rate equations, and the temporal behavior, intensity dependence, and pressure dependence were found to be consistent with a simple kinetic model. The quenching coefficient of NO by N2, Ar, and N2O were determined in this analysis to be qN2 = (2.7±0.8)×10−11 cm3 sec−1, qAr = (6.6±1.4)×10−11 cm3 sec−1, and qN2O = (1.5±0.4)×10−10 cm3 sec−1. Finally, dramatic changes in the spectral distribution of the ultraviolet NO fluorescence due to collisions with He were observed, which contrasts with the absence of spectral redistribution in collisions involving N2, Ar, and N2O.

A study of VUV fluorescence and lasing in electron beam excited xenon

An experimental and theoretical study of VUV radiation in electron-beam excited xenon was undertaken. Fluorescence was observed with efficiencies approaching 50 percent for pressures above 1 atm and modest cell currents (≤ 5 A/cm2). Lasing was also observed at pressures ≥ 2 atm but the presence of premature laser pulse termination reduced efficiencies to < 1 percent. Several possible causes of this premature pulse termination were investigated.

VUV fluorescence from the UV-multiphoton dissociation of chlorinated methanes

Several chlorinated methanes (CCl4, CHCl3, CH2Cl2, CH3Cl, (CH4) and Cl2 were photolyzed with 193 nm ArF laser light. Upon irradiation of CCl4, CHCl3 and CH2Cl2 vacuum UV emission is observed which is attributed to radiation from excited Cl2*. The excited molecules are probably formed by sequential absorption of two photons via a dissociative state of the parent molecule.

Dispersed fluorescence observations of the CO(A 1Π→X 1Σ+) transitions from photodissociation of CO2

Spectra from 1500 to 1800 Å have been obtained showing CO(A 1Π→X 1Σ+) fluorescence arising from photodissociation of CO2. A line emission source provided the incident photons at 15 selected wavelengths from 449 to 955 Å. Analysis of the spectra confirms that the CO(A 1Π→X 1Σ+) band system dominates the vuv fluorescence at incident photon wavelengths from 700 to 923 Å. Vibrational populations of the CO(A 1Π) electronic state for v=0, 1, and 2 are seen to follow Poisson distributions. The onset of dissociation to CO(A 1Π)+O(1D) at 801 Å, the threshold for that process, is apparent from the relative increase in the A 1Π (v=0) population. At the minimum energy required to produce CO(A 1Π)+O(1S), no increase in the v=0 population is observed. Absolute partial cross sections for production of the CO(A→X) bands at incident photon wavelengths of 901.2 and 923.2 Å are reported. The data also show the electronic transition moment to decrease linearly with the r centroid to ∼1.25 Å, consistent with previously established results; however, at larger values of the r centroid the moment increases. The fluorescence observed using incident photons at still shorter wavelengths, 449 Å&amp;lt;λ&amp;lt;500 Å, consists primarily of atomic carbon lines, with an upper limit for the cross section for production of CO(A 1Π) of ∼0.07 Mb.

Sustainer enhancement of the VUV fluorescence in high-pressure xenon

A low-energy (20-60 kV), high-repetition-rate (∼60-Hz) electron-beam system is used to excite xenon at pressures up to 600 psi. The salient features of the temporal and spatial measurements of the VUV fluorescence, corresponding to the1Σ g +-3,1 \Sigma_{u}+ transitions in xenon, are presented. Experimentally, it is found that in a regime where more than 20 percent of the electron-beam energy incident on the gas is converted to VUV fluorescence, the maximum incremental efficiency due to the addition of a sustainer discharge is about 7 percent. A kinetic model is described which attributes the low sustainer efficiencies to electron interactions with the excited state populations.

A model for electron-beam excited VUV fluorescence from xenon

A model for the vacuum ultraviolet emission from electron-beam excited xenon dimers is proposed. The distinguishing feature of the model is its use of two different three-body formation ratesfor the singlet and triplet states of the xenon molecule which have spontaneous decat times of 4 nsec and 16 nsec respectively. Excelent agreement with experiment is found over a very wide range of pressures and currents.