R.A. Back

The decomposition of formic acid vapor by infrared radiation from a pulsed HF-laser

Abstract The decomposition of formic acid vapor by 2.783 μm infrared radiation from a pulsed HF laser has been studied at pressures from 1.6 to 44 torr temper

Fluorescence of ammonia-d3 from its first excited singlet state☆

Abstract Structured emission spectra have been observed from ND 3 excited at 2139 A and 2144 A. The emission is short-lived (τ 1 2 −10 s) and has been assigned to the ND 3 ( A ) → ND 3 ( X ) fluorescence transition.

A comment on the rotational isomerization of ethylene

Abstract Recent spectroscopic measurements have established a torsional barrier in ethylene of 95.5 kcal/mol, ≈ 30 kcal/mol higher than the activation energy for thermal cis-trans isomerization. It is shown that this difference can be explained in terms of a modest (11%) increase in the Cue5f8C bond length as the molecule is twisted through 90°, and its effect on the potential-energy surface for the isomerization.

The decomposition of ethanol vapour by infrared radiation from a pulsed HF-laser

Abstract The decomposition of ethanol vapour induced by infrared radiation from a pulsed HF-laser has been studied as a function of pressure. At high pressures, above 10 torr, the main primary processes appear to be: C 2 H 5 OH → H 2 + CH 3 CHO, C 2 H 5 OH → C 2 H 4 + H 2 O, C 2 H 5 OH → CH 3 + CH 2 OH in a ratio of 3:2:1 which is independent of pressure. At low pressures the process yielding C 2 H 4 and H 2 O becomes dominant. The results suggest that the high pressure behaviour involves a “thermal” decomposition with collisional processes dominating, whereas at low pressures the decomposition is due to multiple photon absorption which at the lowest pressures approaches a collision-free unimolecular decomposition.

Theory of lineshapes in resonance scattering spectra

Abstract The form of the emission spectrum induced by scattering of a generalised light pulse from a single resonance is calculated from the known time dependence of such scattering. It is shown that the intensity of the scattered light is given by | F (ν)| 2 = | G ai (ν)| 2 | S (ν)| 2 , where F (ν), S (ν), and G ai (ν) are the probability amplitudes of the scattered light, the excitation source and the resonance at frequency ν. The implications of this equation in the interpretation of scattering experiments is discussed. In particular, defining resonance Raman scattering as that scattering in which the scattered light has the same bandwidth as the exciting light, it is concluded that although resonance Raman and resonance fluorescence spectra correspond to limiting cases of resonance scattering spectra, spectra in intermediate cases cannot be resolved into a sum of a Raman spectrum and a resonance fluorescence spectrum.

Laser initiated thermal reactions and isotopically selective decomposition of 1,1,1-trichloroethane

Abstract Multiphoton decomposition studies of 1,1,1-trichloroethane (TCE) with a pulsed CO2 laser are reported. Pressure-independent decomposition, suggesting a collisionless process, was found for fluences of 11 J/cm2 with 60 ns pulses, focused to a waist of 1.2 mm, for pressures up to 8 kPa. After 1300 pulses a 50% decomposition of a 16 cm3 sample was observed. However, isotope separation studies with dilute mixtures of TCE-d3 in TCE-h3 showed no selectivity, which is inconsistent with a collisionless process, and suggest a thermal reaction. Similar studies with a shorter, 10 ns, pulse of the same total fluence, on the other hand, indicate that selectivities greater than 104 can be expected. The implications of these apparently conflicting results for laser isotope separation of deuterium are discussed.

Infrared induced reactions in methanol-bromine mixtures: A negative result☆

Abstract Suggestions have been made that infrared induced reactions in CH 3 OH-Br 2 mixtures can lead to isotope separation. A careful quantitative study has been made of limiting quantum yields of the infrared induced reactions in such mixtures and they are found to be very low, φ ⩽ 10 −3 , both with radiation from a laser and from a black body source.

The emission spectrum of the 1B1(π*-n+) state of 1,2-cyclobutanedione excited at 488.0 nm

Abstract The spectrum of the emission from the 1 B 1 (π * -n + ) state of 1,2-cyclobutanedione excited at 488.0 nm has been measured. Wavelengths and vibrational assignments are reported for 24 bands between 490 and 550 nm, 12 of which can be identified with hot bands in the absorption spectrum. Prominent bands in the emission spectrum are associated with excitation of V 8 , the symmetric in-plane carbonyl bend (281 cm −1 ); v 12 , the asymmetric carbonyl wag (488 cm −1 ); and v 7 , a symmetric ring distortion (522 cm −1 ). Sequences in v 13 , the ring-twisting vibration, are also prominent; the initial excitation lies in the 13 3 3 absorption band, while the emission shows intensity maxima for v 13 = 0 and 2, and a bimodal vibrational relaxation is suggested.

The decomposition of ethylene by pulsed CO2 laser radiation at pressures from 500 to 3000 torr and the use of the 2 C2H4⇄cyclobutane equilibrium as an internal thermometer

The decomposition of ethylene by pulsed, unfocussed CO2-laser radiation has been studied at pressures from 500 to 3000 Torr, using the P(14) line of the 10.6μm band (v=949.48cm−1) at incident fluences from about 0.1 to 1.0J/cm2. Major products in order of decreasing importance were 1,3-butadiene, acetylene, ethane, propane, 1-butene and methane. These are known products of the thermal free-radical chain decomposition, and it is concluded that the laser-induced decomposition under our conditions is a transient bulk thermal reaction occurring in a thin disc of heated gas close to the entrance window of the reaction vessel at temperatures ranging from about 1000 to 1500K. As in the thermal decomposition, cyclobutane was observed to be a minor product, which in a sequence of laser pulses approached a final constant concentration. The possibility that this corresponded to an equilibrium concentration at some “effective” reaction temperature was explored. Computer simulation was used to model the accumulation of cyclobutane in the system, both in a single pulse and in a sequence of pulses, and predictions of this model were compared with experiment. It was concluded that cyclobutane could be used in this way as an approximate internal thermometer, within certain limits. Mechanisms of formation of the free-radical chain products are discussed. It is concluded that the chains are initiated by the bimolecular disproportionation reaction, 2C2H4 → C2H3+C2H5, and that secondary initiation by dissociation of the product, 1-butene, becomes increasingly important as the reaction proceeds, leading to autocatalysis. It is further concluded that the radical chain decomposition in this system is a transient process occurring in a brief time interval following the short laser pulse (FWHM=110ns), and is far from steady-state conditions.

Thermal Reactions Of Ethylene Induced By Pulsed Infrared Laser Radiation

The decomposition of ethylene induced by a pulsed infrared CO2 TEA laser has been explored at pressures from 500 to 3000 Torr, using the strongly absorbed P(14) line at 949.5 cm-1. Under these conditions the reaction zone is a thin disc at the front window of the reaction vessel, and the characteristics and behavior of this thin-disc reactor are explored.