Multiphoton Absorption of 12CF3I,13CF3I, CF3Br and SF6 by a HighPressure CO2 Laser

An apparatus for observing the IR multiple-photon resonances of jet-cooled polyatomic molecules using the optothermal technique is described. For experiments in the spectral region 9/11 μm, a commercially available, high-pressure CO2 laser with an improved wave number resolution (∼0.05 cm−1) is used as the pulsed radiation source. Although a moderate pumping capacity is available, high intensity, supersonic beam pulses are obtained by an expressly developed nozzle source. A superconducting bolometer with a responsivity of ∼2×104 V/W and an effective time constant of ≂60 ns (for radiation) has been developed to detect the energy flux carried by the irradiated molecular beam. The time-resolved detector signal is then passed through an analogic processor which separates the fast, small amplitude component generated by the irradiated molecules from the larger component due to the unexcited beam. After further amplification, the useful signal is digitized and stored in a computer to be averaged over several repetitions and then time integrated. A detector input NEP of ∼1.5×10−12 W/(Hz)1/2 is obtained in a single laser pulse. This figure can be further reduced by averaging. Complementary information such as the laser fluence, the intensity of the molecular beam pulse, the optoacoustic signal from a reference gas, as well as the time-of-flight of the molecules can be simultaneously monitored. Part I of this article gives a general description of the apparatus, while Part II gives a detailed description of the bolometer and the related electronics.

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Analysis of SF6 quasicontinuum states. II

Angelié¹

1993

The Journal of Chemical Physics

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The IR fluorescence data of SF6 excited in its quasicontinuum (QC), reported in paper I [C. Angelié, J. Chem. Phys. 96, 8072 (1992)], are analyzed in comparison with IR multiphoton absorption (IRMPA) data, either in the nanosecond or in the picosecond regimes. A general description of the QC transition matrix elements is first derived. All the following features must be taken into account: multiple rovibrational transitions, perturbative redistribution onto secondary resonances, dissipative redistribution onto the whole energy shell, and intramolecular mode inhomogeneity producing a frequency dispersion. The intramolecular couplings obey a hierarchy Vk versus the number k of quanta exchanged, with an effective density of couplings ρk. A model without arbitrary parameters settles that the parameter γ=πρ<V2≳ can be shared between a dissipative part γd∼1–1.5 cm−1 (for E∼10 000–15 000 cm−1), corresponding to the orders k≥5, and a perturbative part γp∼2–15 cm−1, corresponding to the lowest orders k≂3 and 4, i.e., the strongest couplings cannot produce the dissipative relaxation. The same model explains that the QC threshold is at E∼4000 cm−1, as demonstrated by Raman experiments, with a half-width γd∼0.21 cm−1, in excellent agreement with picosecond spectroscopy giving a T1 relaxation time of ∼11 ps. The parameter γ is also extracted from IRMPA cross sections. It is found from these data that γ(E) increases from 0.25 to 15 cm−1 when E increases from 4000 to 34000 cm−1. Finally, picosecond data, showing a spectacular enhancement of the number of photons absorbed for short pulses of duration τL∼30 ps, are fully explained by the previous QC description: Energy is mainly absorbed selectively in the ν3 mode, producing a dramatic enhancement of the Rabi width. Then, all known data on the SF6 QC can be incorporated in a unique framework, likely generalizable to other molecules.

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Multiphoton Absorption of ¹²CF₃I,¹³CF₃I, CF₃Br and SF₆ by a HighPressure CO₂ Laser

Cited by 4 publications

References 15 publications

Multi-photon excitation of a beam of SF6 molecules pumped and probed by cw CO2 lasers

Multi-photon excitation of a beam of SF6 molecules pumped and probed by cw CO2 lasers

Apparatus for the study of the IR multiple-photon resonances of polyatomic molecules via the optothermal technique. I. Description of the apparatus

Analysis of SF6 quasicontinuum states. II

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