Detection of transient laser magnetic resonance signals in pulsed CO2 laser irradiation of SF6/H2O and SF6/NO mixtures

A technique for detecting transient far-infrared laser magnetic resonance (LMR) signals, induced by pulsed CO₂ laser photolysis, has been developed. The method is illustrated with preliminary data from photolysis of SF₆ containing H₂O or NO. In the former mixture. OH(X ²Π) formation and decay is observed, while in the latter, a transient decrease in NO(X ²Π) concentration followed by a return to the original value is observed. Possible physical and chemical mechanisms are discussed.     

Vibrational energy transfer in CH2F2

Infrared fluorescence has been observed from the ν₁, ν₆, 2ν₉, ν₈ and ν₄ levels of CH₂F₂ following excitation by a 9.6 μ Q-switch CO₂ laser. All the observed states exhibit a single exponential decay rate of approximately 44 msec^?1 torr^?1. The rare gas dependence of this rate has also been measured and found to be up to 20 times slower than the rate for the pure gas. Measurements of the risetimes of the observed fluorescence signals yielded an upper limit of 5 μsec at 1 torr for the ν₁, ν₆ and ν₈ levels. The 2ν₉ and ν₄ risetimes were effectively instantaneous under the experimental conditions that prevailed. The relative magnitudes of the measured rate are discussed in terms of existing V-T/R theories and collisional energy transfer processes.     

Spectral properties of the diffuse violet band in Na2 emission: key to the understanding of the excitation mechanisms

The diffuse violet band in Na₂ corresponding to the 2 ³Π_g → a ³Σ⁺_u transition has been studied using the laser-induced fluorescence (LIF) method. In two experiments the violet emission has been observed following a two-step excitation of sodium vapour with either pulsed or cw laser light. Various spectral shapes of the band have been observed depending on excitation wavelength and mode of laser operation. Quantum-mechanical simulations explained the observed spectra in terms of population of individual rovibronic levels in the 2 ³Π_g state. This information has enabled us to analyse the detailed mechanisms of excitation of the 2 ³Π_g state under various experimental conditions.     

Stimulated Raman scattering from H2+ ions

The observation of optical signals due to light scattering from H₂⁺ molecular ions is reported and discussed. By focusing Q-switched 1064 nm Nd : YAG laser light in H₂ at 20 bar, optical frequencies have been generated. They are assigned partly to stimulated Raman scattering from H₂⁺ and partly from H₂ molecules. The scattered radiation (Stokes-shifted 1064 nm beam) was transformed into visible light via higher-order four-wave mixing processes. The frequencies thus obtained could be observed with the naked eye if the scattered light was dispersed and projected on a white screen. The temperature of the gas in the focal region of the laser beam was estimated to be 1030 ± 300 K.     

Photosensitized charge injection into TiO2 particles as studied by microwave absorption

The fluorescence of rose bengal is quenched with 96% efficiency when this dye is adsorbed on TiO₂ particles. Microwave measurements on suspensions of these dye-coated particles have shown large absorption signals when the suspension is irradiated with pulses of 532 nm light from a laser. The microwave absorption is attributed to mobile charge carriers injected into the TiO₂ from the excited singlet state of the dye. The observed rise time of the signal indicates no intermediate surface state of greater than 2 ns lifetime. Various characteristics of the observed signals are compared with those from TiO₂ single crystals.     

Direct observation of rotational energy transfer in ammonia by time-resolved infrared—microwave double resonance

An N₂O laser is used to pump the ground vibrational state (8,7) inversion doublet of ¹⁴NH₃ while simultaneously monitoring other ground state doublets. Time-resolved rotational energy transfer signals are observed in accordance with known selection values. Absolute rates of rotational energy transfer processes are estimated.     

N2O5 photolysis products investigated by fluorescence and optoacoustic techniques

Pulsed laser photolysis of N₂O₅ near 290 nm coupled with fluorescence detection (calibrated by NO₂ photolysis) showed that the O(³P) quantum yield is ≤0.1. A pulsed laser optoacoustic technique in a flow tube (ca. 6 torr of N₂) was tested by photolysis of NO₂ and then applied to N₂O₅. Nitric oxide was added to react with NO₃ free radical and the resulting increase in the optoacoustic signal confirmed the presence of NO₃ free radicals. Based on the relative optoacoustic signals observed for NO₂ and N₂O₅, the quantum yield for NO₃ production is 0.8 ± 0.2.     

Diagnosis of Active Species in Reactive Acetylene Plasma by Laser-Ionization Molecular-Beam Time-of-Flight Mass Spectrometry

This work reports a new laser molecular beam time-of-flight mass spectrometry (LMB-TOFMS) instrument built at Plasma Spectroscopy Lab in Dalian University of Technology (DUT). The preliminary investigation of the intermediate species distribution of acetylene plasma in an atmospheric-pressure fast-flow nanosecond-pulsed dc-discharge has been carried out. The C_xH₂ species (x = 4,6,8), believed to be the dominant source of negative ions in C₂H₂ plasma, have been detected with relatively intense peak signals and some complex radical species like C₇H₇, C₉H₅, C₁₀H₅ are also observed. The addition of Ar gas significantly influenced the detection of the intermediate species. This is might be due to the metastable Ar^* with energies of 11.5 and 11.7 eV and a long lifetime. The potentialities of LMB-TOFMS for hydrocarbon plasmas diagnosis have been discussed.     

Time-of-flight mass spectroscopy of femtosecond and nanosecond laser ablated TeO2 crystals

Single-pulse femtosecond (fs) (pulse duration ～200 fs, wavelength 398 nm) and nanosecond (ns) (pulse duration 4 ns, wavelength 355 nm) laser ablation have been applied in combination with time-of-flight mass spectrometer (TOFMS) to analyze the elemental composition of the plasma plume of single-crystalline telluria (c-TeO₂, grown by the balance controlled Czochralski growth method). Due to the three-order difference of the peak intensities of the ns and fs-laser pulses, significant differences were observed regarding the laser-induced species in the plasma plume. Positive singly, doubly and triply charged Te ions (Te⁺, Te₂⁺, Te₃⁺) in the form of their isotopes were observed in case of both irradiations. In case of the ns-laser ablation the TeO⁺ formation was negligible compared to the fs case and there was no Te trimer (Te₃⁺) formation observed. It was found that the amplitude of Te ion signals strongly depended on the applied laser pulse energy. Singly charged oxygen ions (O⁺) are always present as a byproduct in both kinds of laser ablation.     

On the investigation of the preparation of inorganic ionic clusters by laser desorption ionization

CeO(H₂O) _n ⁺ cluster ions, withn as large as 20, are generated by laser desorption ionization of a frozen aqueous solution of CeCl₃,7H₂O. A simple dynamic model developed previously can qualitatively account for experimentally observed threshold behavior, and the time- and fluence-dependence of ion signals.Dedicated to Professor Jiaxi Lu on the occasion of his 80th birthday.     

SF6-sensitized dissociation of UF6 with a pulsed CO2 laser

The dissociation of UF₆ sensitized by SF₆ excited with a pulsed CO₂ laser in the presence of H₂ and CO as scavengers has been investigated. In the SF₆-UF₆-H₂ system the dissociation yields have been determined as a function of the laser frequency, the fluence, and H₂ partial pressure. A maximum dissociation yield has been found at a laser frequency of 935 cm^?1. No obvious dissociation of UF₆ was observed in the UF₆-SF₆ system without F-atom scavengers.     

Multiphoton dissociation/ionisation of dimethyl sulphide (CH3SCH3) at 355 and 532 nm

Multiphoton dissociation/ionization has been studied for CH₃SCH₃ at 355 and 532 nm using a time-of-flight mass spectrometer. The major ion signals observed at 355 nm are C⁺, CH₃ ⁺, HCS⁺, CH₂S⁺, CH₃S⁺ and CH₃SCH₃ ⁺. Power dependence studies at 355 nm show a (2+1) REMPI process for the formation of parent ion. Peaks atm/e = 46, 47 and 61 show two-photon laser power dependence whereasm/e = 15 and 45 peaks show four-photon dependence. However, in 532 nm photo-ionisation, no parent ion signal is observed. A peak atm/e = 35 corresponding to SH₃ ⁺ has been observed. SH₃ ⁺ has been suggested to originate from CH₃SCH₂ ⁺ via a cyclic transition state. Photoionisation results of CH₃SCH₃ have been compared with those of CH₃SSCH₃, at these two wavelengths.     

Laser driven synthesis of BHCl2 from BCl3 and H2

Mixtures of BCl₃ and H₂ have been irradiated by a pulsed CO₂ laser yielding BCHl₂ and HCl as the only products. Approximately 50% conversion of the reagents was obtained with a quantum efficiency of 122 10.6 μ photons per BCHl₂ molecule. The product yield is observed to scale with laser power as (P/P₀)^1.26.     

Competition between multiphoton fluorescence and multiphoton ionization in NO

A nitrogen laser pumped tunable dye laser has been used to observe the three-photon ionization of NO through a two-photon resonance with the C²II state. Fluorescence is also observed from this state. The wavelength dependence of both signals have been measured. A reaction mechanism is postulated, which includes the initial two-photon excitation of the C²II state as the rate-limiting step. This mechanism predicts the observed second-order intensity dependence of the ionization signal and shows that the simple rate equation treatment is valid in this system.     

CO2 laser induced isomerization and photodissociation of 1,2-dichloroethylene

Infrared photoisomerization of both trans- and cis-1,2-dichloroethylene molecules sensitized by SF₆ has been observed by using a CO₂ laser. The direct infrared photoisomerization has also been observed for the trans molecule. The reaction rate of the sensitized photoisomerization increases by 6 times as compared with the direct process. The dielectric breakdown induced by an intense laser pulse enhances a dissociative reaction.     

Vibrational energy transfer in CH3I

Previous works have reported vibration—vibration and vibration—translation transfer rates in the methyl halides. Using the technique of laser induced infrared fluorescence we have studied energy transfer in the concluding member of this series, CH₃I. Following excitation by resonant lines of a Q-switch CO₂ laser, infrared fluorescence has been observed from the v₂, v₅ as well as the 2v₅, v₁, v₄ vibrational energy levels of CH₃I. All the observed states exhibit a single exponential decay rate of 23 ± 2 ms^?1 torr^?1. Measurements have also been made on deactivation of the various modes by rare gases. The risetime of the v₂, v₅ levels was found to be approximately 101 ± 20 ms^?1 torr^?1, while that of the 2v₅, v₁, v₄ levels was approximately 225 ± 45 ms^?1 torr^?1. Fluorescence was not detected from the v₃ level. These results are discussed in terms of SSH type theoretical calculations, and comparison is made with the results obtained for other members of the methyl halide series, namely CH₃F, CH₃Cl and CH₃Br.     

Transient excited singlet state absorption in POPOP and dimethyl POPOP

Transient excited singlet state absorption (ESSA) has been measured in POPOP solution in ethanol and dimethyl POPOP solution in toluene at room temperature in the region 4580–6880 Å using a nitrogen laser and nitrogen laser pumped dye laser. Extensive absorption with several submaxima and shoulders, which represent the vibrational structure, has been observed in both molecules in the region covered in the present study. Energy level schemes of the two molecules have been obtained with the help of the ground state absorption and fluorescence spectra recorded for the purpose. The observed structure in the ESSA has been tentatively interpreted to be due to transitions between the different vibrational levels of the lowest excited state S₁ and two other upper singlet electronic states S₃ and S₄ or S_n. The occurrence of transitions from the higher vibrational levels of S₁ in addition to those from its lowest vibrational level could be understood on the basis of the fact that the pump and the probe beams are made to overlap for a period of ∼ 6–7 ns during which they interact simultaneously with the dye molecules. A plausible explanation for the relative variation in the strength of different transitions from S₀ as well as the observed difference (or otherwise) in the strength of these transitions relative to those of S₁ in each molecule is given on the basis of parity/symmetry selection rules. The experimental results obtained and the transition assignments made are being reported for the first time.     

Characterization of SiN and other transient species in a silicon tetrachloride–nitrogen discharge

Infrared diode laser absorption spectroscopy has been used to detect the A–X electronic spectrum of diatomic SiN in a SiCl₄/N₂ plasma. The intensity of the SiN transitions with respect to the ratio of SiCl₄:N₂ flow rate was investigated. A correlation between intense signals of SiN and the optimal growth conditions for a-SiN films reported in literature was found. This observation lends support to the importance of diatomic SiN as a film precursor. The electronic emission spectra of silicon-containing transient species were also recorded in the plasma. A brief discussion of the plasma reaction pathways of SiN is presented.