F-center Laser Research Articles

IR-laser induced interconversions of hydrogen-bonded dimers in cryogenic matrices

IR-laser induced interconversions of hydrogen-bonded alcohol dimers have been studied in cryogenic matrices by means of FTIR spectroscopy and a tunable F-center laser as photolysis source. Four different conformational forms of the open chain methanol dimer have been identified in solid nitrogen matrices. These conformations have been interconverted by selective vibrational excitation into the OH stretching absorption bands.

Infrared spectrum and autodetachment dynamics of NH−

The infrared vibration-rotation spectrum of NH− has been obtained by autodetachment spectroscopy in a coaxial laser–ion beam spectrometer. Transitions from the v=0 to v=1 vibrational levels were excited with an F-center laser, and subsequent autodetachment from the v=1 levels was observed. The apparatus resolution was better than 20 MHz, allowing the resolution of the fine structure and Λ-doubling transitions. The linewidths of the autodetachment resonances revealed some of the dynamics of the autodetachment process. The autodetachment rates were, in general, much greater for the upper Λ-doublet levels of NH−(v=1) than for the lower levels. In addition, the increase of the autodetachment rate with rotational energy for the upper levels was much faster than would be predicted if vibrational autodetachment were the primary detachment mechanism. It therefore appears that rotational-electronic coupling plays an important role in this system, and the differences in the Λ-doublet autodetachment rates are explained in terms of this mechanism.

Resonant rotational energy transfer in HF

A molecular beam apparatus has been used to study resonant rotational energy transfer between HF molecules in the ground vibrational state. A low temperature beam formed from the supersonic expansion of a gas mixture containing 1% HF in helium is attenuated by HF molecules with a thermal energy distribution. State selection of the molecular beam is accomplished by using an F-center laser in conjunction with a bolometer detector. The attenuation measurements show effects which are attributed to resonant rotational energy transfer. The results have been analyzed using a kinetic model to estimate the relevant cross sections. The first order dipole allowed process HF(J=0)+HF(J=1) →HF(J=1)+HF(J=0) is found to have a cross section of ∼300 Å2. The results also suggest that the second order cross sections, having ΔJ=2, are about 40 Å2.

Molecular beam infrared spectra of dimers formed from acetylene, methyl acetylene, and ethene as a function of source pressure and concentration

Infrared spectra have been obtained for dimers formed from acetylene, methyl acetylene, and ethylene using a wide variety of free jet source conditions. An F-center laser was used to excite various C–H stretch fundamentals in the range 3000–3300 cm−1. Rotational structure was resolved in the spectra of both acetylene and methyl acetylene dimers when dilute mixtures of either gas in helium were expanded from high pressures. The spectrum of the ethylene dimer showed no such structure even when formed in very dilute mixtures. These results contradict previous suggestions that the vibrational relaxation lifetime is uniformly short for all polyatomic dimers.

Solid state materials for CW tunable infrared lasers

The different techniques and materials involved in IR tunable CW coherent light sources are reviewed encompassing semiconductor types as well as vibronics ones in insulating crystals. Recent results obtained at CNET on MgF2:Ni2+ laser are presented and compared to results for F-center laser materials.

Sub-Doppler resolution infrared spectroscopy of the acetylene dimer: A direct measurement of the predissociation lifetime

Rotationally resolved, infrared predissociation spectra have been obtained for (C2H2)2 using a molecular beam apparatus in conjunction with a single mode F-center laser operating near 3300 cm−1. Several well separated bands are seen in the low resolution spectra, possibly indicating that more than one stable dimer structure exists. The high resolution spectra indicate that the widths of the individual rovibrational transitions, and hence predissociation lifetimes, depend on the band excited. The lifetimes obtained from the widths vary from 80×10−9 to 1.6×10−9 s.

Infrared double resonance spectroscopy of V-T, R relaxation of HF(v=1): Direct measurement of the high-J populations

The V-T, R relaxation of HF(v=1) by HF is studied by infrared pulse-probe transient absorption measurements using a tunable F-center laser. It is found that a substantial fraction of the relaxation occurs through the high-lying rotational levels of v=0. The results indicate that the states J=10–14 comprise ∼20%–40% of the population in the total relaxation, with the distribution increasing rapidly with decreasing J in this range. The observed signals, corrected for cascade, correspond to the fractions: J=10, ∼10%–40%; J=11, ∼4%–10%; J=12, ∼2%–6%; J=13, ∼1%–2%; and J=14, ∼0.05%–0.35%. In addition, the V-T, R rate determined here, k=1.46±0.15×10−12 cm3 molecule−1 s−1 reconfirms previous double resonance results. Also, studies involving buffer gas addition (Ar) give no indication of ‘‘bottleneck’’ effects on the high J’s. In fact, the R–R, T rates always exceed the V-T, R rate by one to two orders of magnitude. For J=11–13, the R–R, T rates range from ∼1.2×10−10 to 6×10−11 cm3 molecule−1 s−1.

Infrared photoacoustic spectroscopy of liquids with an F-center laser

Description des appareillages et methodes de mesure utilisees en spectroscopie photoacoustique, utilisant un laser a centre F et permettant la detection de faibles absorptions en IR dans les liquides

Laser Stark spectroscopy of carbon dioxide in a molecular beam

Laser Stark spectroscopy has been performed for CO 2 molecules in a bolometrically detected supersonic beam, using a tunable infrared F-center laser to excite rovibrational transitions in the 3715-cm −1 “ ν 1 + ν 3” band of 12C 16O 2. The very weak Stark effect of CO 2 is amply resolved by realizing an effective spectroscopic linewidth of 2 MHz and by using uniform electric field strengths in excess of 200 kV cm −1. Stark spectra for the R(0) and P(2) transitions enable evaluation of the isotropic part ( α n ) and anisotropy (Δ α n ) of the static molecular polarizability in both excited ( n = v) and ground ( n = 0) vibrational states. Direct observables are Δα v Δα 0 = 1.021(8) ; (α v - α 0) Δα v = 0.012(6) ; Δα v = 2.65(18) A ̊ 3 , the last result being 12% greater than predicted. The results test the relative magnitudes of electronic and vibrational contributions to the static polarizability and indicate the way in which the polarizability tensor varies from one vibrational state to another.

Rotational relaxation of HF in a free-jet expansion of dilute HF–He mixtures: Information content on state-to-state rate constants

An F-center laser–molecular beam spectrometer has been used to provide detailed information concerning the translational and rotational relaxation resulting from the free-jet expansion of dilute mixtures of hydrogen fluoride in helium. The observed non-Boltzmann rotational state distributions are modeled using an empirical kinetic approach and are shown to contain information concerning the individual state-to-state rate constants for HF–He.

Photoacoustic spectroscopy of the ν_3 band of HCN

All fundamental transition frequencies to the 25 lowest rotational levels of the ν3 band of HCN have been measured for the first time to our knowledge using a photoacoustic spectrometer consisting of an F-center laser, a photoacoustic absorption cell, and an infrared wavemeter. The 49 transitions were measured at an HCN pressure of only 20 mTorr and had an rms deviation of 0.002 cm−1 from the calculated values. Effects of pressure broadening and pressure shifts were examined. In addition, the techniques of intracavity photoacoustic spectroscopy and frequency-modulated photoacoustic detection have been demonstrated for studying transitions in this band at low pressure.

Principles of Lasers

The characteristics of the individual lasers were studied and their applications in analytical chemistry were given. Laser properties: the gain medium, the pumping mechanism, and the resonator design were discussed. Gas lasers, argon and krypton; solid state lasers, ruby, yttrium, aluminum garnet, semiconductor diode, F-center laser; and dye lasers, flashlamp pumped, nitrogen exchanger, CW dye laser; were all compared. The free electron laser is now under development. 8 figures. (DP)

Infrared optogalvanic spectroscopy in the helium positive column using an F-center laser

Using optogalvanic spectroscopy with a tunable cw F-center laser, we have detected helium transitions from the n=4 to the n=6 states between 2.61 μm and 2.7 μm. The observed sensitivity is in agreement with the predictions of a steady state model of the positive column discharge.

Investigation of the spectral characteristics of a pulsedF2-center laser tunable in the range 1.1–1.26μ

A description is given of a simple resonator system in which single-frequency stimulated emission was achieved from a pulsed F-center laser. The average line width due to fluctuations of the lasing frequency was 0.01 cm–1 in the 1.1–1.26 μ spectral range. Investigations were made of the reasons for these fluctuations and numerical estimates were made of the different factors responsible for these fluctuations.

Linear and nonlinear optical properties of Ca 3(VO 4) 2

Single crystals of Ca 3 (VO 4) 2 have been grown by the Czochralski method. The refractive indices show that the crystals are phasematchable between 1.72 μm and 2.94 μm with favourable values for acceptance angle and spectral width of the incident laser beam in the entire phasematching range. The nonlinear optical coefficients have been measured by the Maker fringe method at a fundamental wavelength of 1.064 μm. They are comparable to those of KDP. Thus the crystals are potentially useful for frequency conversion of broadband or tunable laser sources as semiconductor and F-center lasers. They may also be installed in high f-number IR image converters.