Excimer Laser Flash Photolysis Research Articles

Infrared spectroscopy of jet-cooled transient molecules

Free radicals are produced by excimer laser flash photolysis of a suitable precursor inside the nozzle of a pulsed slit jet and cooled by supersonic expansion from the slit. The infrared spectra of the radicals are then probed by a single-frequency color-center laser. This technique has been tested on the NH 2 radical.

Absolute rate constants for the reaction of SiH with hydrogen, deuterium and silane

Silylidyne, SiH, has been generated by excimer laser flash photolysis and its concentration monitored in real time by laser-induced fluorescence. Removal rates for SiH by hydrogen, deuterium and silane have been determined at 5 Torr total pressure in helium at ambient temperature. Silylidyne reacts gas-kinetically with silane, but considerably more slowly with hydrogen and deuterium.

ChemInform Abstract: ArF Laser Flash Photolysis of Phenol and Anisole

Absorption spectra of phenoxy radical in the gas phase are observed in the range of 210–300 nm in the ArF excimer laser (193 nm) flash photolysis of phenol and anisole. An intense absorption band is obtained around 230 nm and a broad band around 270–300 nm. In the photolysis of anisole, simultaneous formation of methyl and phenoxy radicals makes it possible to determine the molar extinction coefficient of phenoxy radical which is determined to be 7800 M−1 cm−1 at 230 nm. The phenoxy radicals are produced in the time range of shorter than 1 ns from phenol, anisole, pentafluorophenol, and pentafluoroanisole on irradiation with the ArF laser light. The photodissociation is not explained in terms of the hot molecule mechanism but of predissociation.

Infrared flash kinetic spectroscopy of HCO

The high resolution infrared spectrum of the CH stretching fundamental of the formyl radical (HCO) has been observed by means of infrared kinetic spectroscopy using 308 nm (XeCl) excimer laser flash photolysis of formaldehyde or acetaldehyde followed by diode or difference frequency laser probing of the transient absorption. The high resolution spectra obtained were assigned and fitted with rotational, spin–rotational, and centrifugal distortion constants. The ν1 band origin is 2434.48 cm−1. New ground-state constants are reported from a least-squares fit combining this ν1 data with previous microwave and FIR LMR measurements.

ArF laser flash photolysis of phenol and anisole

Absorption spectra of phenoxy radical in the gas phase are observed in the range of 210–300 nm in the ArF excimer laser (193 nm) flash photolysis of phenol and anisole. An intense absorption band is obtained around 230 nm and a broad band around 270–300 nm. In the photolysis of anisole, simultaneous formation of methyl and phenoxy radicals makes it possible to determine the molar extinction coefficient of phenoxy radical which is determined to be 7800 M−1 cm−1 at 230 nm. The phenoxy radicals are produced in the time range of shorter than 1 ns from phenol, anisole, pentafluorophenol, and pentafluoroanisole on irradiation with the ArF laser light. The photodissociation is not explained in terms of the hot molecule mechanism but of predissociation.

Rate constant measurements of reactions of ethynyl radical with hydrogen, oxygen, acetylene and nitric oxide using color center laser kinetic spectroscopy

The rate constants for the reactions of the ethynyl radical (C/sub 2/H) with H/sub 2/, O/sub 2/, C/sub 2/H/sub 2/, and NO have been measured by following the time decay of an C/sub 2/H infrared transient absorption line originating from the ground vibronic state using color center laser spectroscopy. For the H/sub 2/, O/sub 2/, and NO reactions, the C/sub 2/H was produced by excimer laser flash photolysis (ArF, 193 nm) of CF/sub 3/C/sub 2/H. In the case of the C/sub 2/H/sub 2/ reaction C/sub 2/H was produced by flash photolysis of acetylene again using the 193-nm ArF excimer line. Excited states of C/sub 2/H, which are abundant with 193-nm photolysis, were relaxed by buffering the photolysis cell with approx. 20 Torr of He buffer and approx. 160 mTorr of SF/sub 6/. Rate constants of 4.2 x 10/sup -11/, 4.8 x 10/sup -13/, 1.5 x 10/sup -10/, and 3.5 x 10/sup -11/ cm/sup 3/ molecule/sup -1/ s/sup -1/ were obtained for the reactions of C/sub 2/H with O/sub 2/, H/sub 2/, C/sub 2/H/sub 2/, and NO, respectively.

Color center laser spectroscopy of vibrationally excited C2H

Infrared transitions involving vibrationally excited C2H radicals produced by excimer laser (193 nm) flash photolysis of acetylene were observed in transient absorption by color center laser probing. Three bands with origins at 3702, 3562, and 3320 cm−1 were found, assigned, and analyzed. All three bands are of 2∑–2∑ symmetry with the lower state of each transition the vibrationally excited (0,0,1) level of the ground electronic state. The frequencies and fitted spectroscopic constants for the three bands are reported. Possible vibronic assignments of the upper states are discussed.

Studies of the amidogen-nitric oxide reaction by infrared kinetic spectroscopy

Infrared kinetic spectroscopy using excimer laser flash photolysis and color center laser probing has been used to study the NH/sub 2/ + NO reaction. The amidogen radical, NH/sub 2/, was produced by ArF photolysis of NH/sub 3/. Infrared absorptions of OH and H/sub 2/O were measured to determine the absolute contributions of the OH and H/sub 2/O product channels. It was found that the OH channel accounts for 13 +- 2% of the reaction. Using two different pairs of NH/sub 3/ and H/sub 2/O lines, we measured values of 0.85 +- 0.09 and 0.66 +- 0.03 for the ratio of H/sub 2/O formed to NH/sub 3/ photolyzed. All of the H/sub 2/O signals exhibit a pronounced induction period suggesting that H/sub 2/O is produced in very high vibrational states. The time evolution of low-lying vibrationally excited and ground vibrational state H/sub 2/O lines is adequately simulated by a model in which a stepwise sequential loss of vibrational energy occurs with quenching cross sections for each step proportional to excess energy.

Color-center laser spectroscopy of transient species produced by excimer-laser flash photolysis

Kinetic spectroscopy based on excimer-laser flash photolysis and color-center-laser (CCL) infrared probing is explored. In simple absorption, the achievable signal-to-noise ratio (S/N) is not satisfactory even though the signal itself (corresponding to >1% absorption) is fairly large. This is due to amplitude fluctuations of the CCL. By using a double-beam detection scheme to balance out these amplitude fluctuations the sensitivity can be improved to the extent that a 1% absorption gives a S/N ∼ 100. In certain situations transient decreases in absorption of the precursor and transient increases in absorption due to final product formation can produce severe interfering signals even in simple systems. This problem is overcome without a major loss in sensitivity by a recently developed 45° magnetic rotation scheme. These points are illustrated with spectra of Br, OH, and NH2.

A time-resolved technique for measuring rates of alkyl radical-atom reactions: The pressure dependence of the CH 3 + H reaction at 504 K

The rate constant for the CH 3 + H reaction has been measured as a function of pressure at 504 K. A new technique was employed, in which the temporal behaviour of both the atom and radical was measured, by resonance fluorescence and UV absorption spectroscopy respectively, following excimer laser flash photolysis of acetone at 193 nm. A minor complication, resulting from the production of small amounts of CH 2 from a two-photon process, was eliminated by studying the atom and radical decays as a function of laser pulse energy.

Spectroscopic characterization of 2-naphthylcarbene

The 308 nm excimer laser flash photolysis of 2-naphthyldiazomethane produces triplet 2-naphthylcarbene (λ max = 362 nm) which decays with the observed pseudo-first-order rate constants (k exptl) of 5.54 ± 0.03 × 10 6; 3.33 ± 0.4 × 10 6; 1.64±0.02 × 10 7; and 3.05±0.4 × 10 6 s -1 in n-pentane, 2,2,4-trimethylpentane (2,2,4-TMP), benzene and Freon 113 respectively. In hydrocarbon solvents the observed decay of triplet 2-naphthylcarbene is correlated with the pseudo-first-order growth of the 2-naphthylmethyl radical (λ max = 378 nm). Direct kinetic measurements of the reaction of triplet 2-naphthylcarbene in 2,2,4-TMP with cyclohexane, styrene, methanol and carbon tetrachloride yielded bimolecular quenching rate constants of 1.48 ± 0.04 × 10 6;4.33 ± 0.1 × 10 7;7.25 ± 0.5 × 10 6; and 3.35 ± 0.07 × 10 6M -1S -1. It is also found that 2-naphthylcarbene reacts with acetonitrile (k q = 5.28 ± 0.1 × 10 5 M -1 s -1) to form a nitrile ylide intermediate with a λ max = 372 nm. These results are interpreted in terms of a rapid singlet-triplet 2-naphthylcarbene equilibrium.

Application of laser photolysis to the study of reactive intermediates

Excimer laser flash photolysis in the study of diphenyl carbene (DPC) reveals the presence of at least three transient species produced by excitation of a precursor at 249 nm in isooctane solution at room temperature depending on the concentration of O 2 present. In an aerated solution, a fast first-order decay assigned to3DPC leads to the formation of an unassigned species which also decays by first order kinetics to produce the observed product Ph 2 CO. In degassed solution, the fast first-order decay of a species to produce3DPC which decays by second-order kinetics to produce the dimeric products observed.