CO2 Laser Lines Research Articles

ChemInform Abstract: THE MULTIPHOTON ABSORPTION AND DECOMPOSITION OF FLUOROFORM‐D: LASER ISOTOPE SEPARATION OF DEUTERIUM

Multiphoton absorption (MPA) studies of fluoroform‐d, a molecule of interest for potential laser based hydrogen isotope separation processes, are reported for CDF3 pressures 0.2–1.3 kPa, and for a variety of 10 μm CO2 laser lines with pulse widths of 2 or 6 ns and fluences within the range 10−3–70 J/cm2. Unlike SF6, no red shift of the MPA spectrum relative to the small signal spectrum was observed at high fluence. Selective multiphoton decomposition (MPD) experiments using the 10R(26) line, 6 ns pulse to excite the CDF3 component in natural‐abundance CHF3 (∼ 150 ppm D/H) at a pressure of 13.3 kPa resulted in the recovery of water enriched up to 30% in deuterium—a measured isotope enrichment of ⩾2000 fold. This demonstrates that a product, highly enriched in deuterium, can be recovered from the selective MPD of fluoroform.

Raman gain in 12-μm NH_3 lasers

We report the first known observation of lasing in NH (3) on the aP(6, 2) transition at 12.26 microm. Emission is obtained by pumping the aR(4, 2) line with the P(34) 9-microm CO(2) laser line. The frequency offset between pump and absorption line center is 5.16 GHz. This pump-emission pair was chosen to minimize the effects of ac Stark shifts and population transfer and allows us to study the Raman processes occurring in this type of optically pumped laser. We have confirmed that lasing takes place by a Raman process and obtained good agreement between theory and experimental measurements of small-signal gain.

Ethylene monitoring by differential absorption with a CO2 waveguide laser

In this paper we report on a differential absorption technique using a CO2 waveguide laser. The method is based on the measurement of the differential absorption between two different frequencies of the same selected CO2 laser line. The sensitivity of the technique depends strongly on the total pressure and has been estimated to be about a few tens of ppb or better over a 1 km path length in C2H4 diluted with air to a total pressure of a few tens of torrs. Its relative simplicity and high sensitivity at low total pressure enable this technique to be applied to gas detection either at high altitude or to local monitoring of a low pressure sample.

High-resolution laser Stark and Fourier transform spectroscopy of DBr at 5.5 μm

A very close coincidence between the P(1) transition of the 1-0 band of D 79Br and a CO laser line has allowed the observation of a DBr laser Stark spectrum. The observed inverse Lamb dips resolve the Br hyperfine structure of the transition, and are used to determine the value of the dipole moment of DBr in the v = 1 state. A high-resolution Fourier transform spectrum of DBr in the region 1750 to 1900 cm −1 has also been recorded and accurately measured. These new data, together with the best previous microwave and infrared measurements, are used to determine molecular parameters for DBr, and also result in improved secondary wavenumber standards for the 5.5-μm region.

LMR spectra of DO2 tuned by the Coriolis-induced avoidance of Zeeman level crossings

In an extension of the study of high-field LMR spectra [H. Uehara and K. Hakuta, Chem. Phys. Lett. 58, 287 (1978); J. Chem. Phys. 74, 969 (1981)], it is found that level mixing due to the Coriolis interaction also induces the avoidance of Zeeman level crossings and brings about unexpected high-field LMR spectral lines. The presence of Coriolis-induced avoidance between 55, F2 Zeeman levels of the (010) state and 54, F1 Zeeman levels of the (001) state of the DO2 radical has been confirmed and LMR spectral lines of the ν2 band of DO2 tuned by the Coriolis-induced avoidance have been identified by use of CO2 laser lines in the 9.8 μm region. The present detection of the Coriolis-induced Zeeman spectrum provides support for the analysis of the Coriolis interaction of DO2 by McKellar [A. R. W. McKellar, J. Chem. Phys. 71, 81 (1979)].

The multiphoton absorption and decomposition of fluoroform-d: Laser isotope separation of deuterium

Multiphoton absorption (MPA) studies of fluoroform-d, a molecule of interest for potential laser based hydrogen isotope separation processes, are reported for CDF3 pressures 0.2–1.3 kPa, and for a variety of 10 μm CO2 laser lines with pulse widths of 2 or 6 ns and fluences within the range 10−3–70 J/cm2. Unlike SF6, no red shift of the MPA spectrum relative to the small signal spectrum was observed at high fluence. Selective multiphoton decomposition (MPD) experiments using the 10R(26) line, 6 ns pulse to excite the CDF3 component in natural-abundance CHF3 (∼ 150 ppm D/H) at a pressure of 13.3 kPa resulted in the recovery of water enriched up to 30% in deuterium—a measured isotope enrichment of ⩾2000 fold. This demonstrates that a product, highly enriched in deuterium, can be recovered from the selective MPD of fluoroform.

High-resolution laser spectroscopy of the ν3 vibration-rotation band of HCOOH

The ν3 (C = O stretching) band of HCOOH has been studied by the techniques of CO laser Stark spectroscopy and tunable diode laser spectroscopy. The coincidence in frequency between the 13–12 P(15) CO laser line and the 110←211 transition of the ν3 band has been found by Stark spectroscopy and confirmed by laser-microwave double resonance. It is found that the Ka = 4 and Ka = 7 levels of the ν3 state are perturbed because of Coriolis interactions with the close-lying ν5+ν9 and ν6+ν9 states, respectively. The band origin, rotational and centrifugal distortion constants, and components of the dipole moment are determined for the ν3 state from the analysis of the unperturbed transitions. The Coriolis coupling constants and information on the interacting states are obtained from the analysis of perturbed transitions.

Changes in fluorescence induced by infrared multiphoton excitation of optically excited SO2

Photophysical properties of excited SO2 were studied by the UV+MPE technique: single-photon UV excitation together with IR multiphoton excitation (MPE). Compared to the fluorescence in the absence of IR irradiation, the fluorescence following MPE showed a decrease of the main fluorescence peaks, accompanied by the appearance of broad fluorescence features to the high-energy side of the excitation frequency. Different CO2 laser lines were about equally effective. The dependence on laser fluence can be accounted for quantitatively using a simple stepladder model for MPE. The cross section for absorption of CO2 laser radiation by SO2, excited to around 32 000 cm−1, was found to be σ = 1.5×10−21 cm2. The effective decay time of the fluorescence with IR-on is increased twofold, compared to that with IR-off. This increase seems to parallel the reported dependence of the lifetimes on excitation frequency. It may be interpreted as due to more efficient coupling of excited vibronic states with highly-excited vibrational states in the ground electronic state.

Efficient production of13C2F4 in the infrared laser photolysis of CHClF2

We report the isotopically selective decomposition of chlorodifluoromethane. Chlorodifluoromethane is used industrially in high volume for the production of tetrafluoroethylene and its polymers; thereby it is an attractive working substrate for a medium scale isotope separation process, both in terms of its price and availability. We have studied the infrared multiphoton decomposition of carbon-13 substituted chlorodifluoromethane molecules present at their natural abundance (1.11%). A well defined CO2 laser pulse (80 ns FWHM) was used and both the yield of carbon-13 enriched product and the net absorption of laser radiation were measured. These measurements were made as a function of substrate pressure (10-800 Torr), CO2 laser line (9P 12–9P 32) and fluence (2–8 J cm−2) and were used to determine the energy expenditure per carbon atom produced (e) at specified product carbon-13 content in the range 30%–96%. The results of these parametric studies were interpreted in terms of the kinetics of multiphoton absorption and dissociation, and allowed an initial optimization of the experimental conditions to minimize e. Optimum results were obtained at 1046.9 cm−1, 69 cm−1 to the red of the12CHClF2 v 9 band center. Irradiation of 100 Torr of chlorodifluoromethane at 3.5 J cm−2 gave tetrafluoroethylene containing 50% carbon-13 for an absorption of 140 photons (0.017 keV) per carbon atom produced. This efficiency compares favourably with existing carbon-13 enrichment technologies and would require an absorption pathlength of only 2 m to absorb half the incident photons.

Laser magnetic resonance of metastable krypton and xenon atoms

Magnetic dipole transitions from the metastable ls5 (3P2) state to the ls4 (3P1) state in atomic krypton and xenon have been detected and measured using the technique of intracavity laser magnetic resonance spectroscopy with CO2 laser lines in the 10 μm region. The excited atoms were produced by a low voltage dc discharge in a flow of helium carrier gas. Precise values for the Kr and Xe ls4– ls5 interval and for the magnetic gj-factor of ls4 in Kr were determined in an analysis of the observed spectra.

Optoacoustic Laser Stark spectroscopy and FIR laser action on CH3Br using a waveguide CO2 laser

The optoacoustic spectrum of CH3Br around 10 μm band lines of a tunable cw waveguide CO2 laser is investigated. Several new infrared absorptions are observed and most of the correspond ing molecular transitions are assigned. Far infra red laser action is reported by pumping with the same CO2 laser: pump offsets are given using the Transferred Lambs dip (TLD) technique. A new FIR laser emission is obtained and assigned. An optoacoustic Laser Stark spectroscopy technique is used to investigate off resonance infrared tran sitions.

Sub-Doppler double-resonance spectroscopy of NH_3 using a tunable-diode laser and a CO laser

Results are given for a Stark-tuned double-resonance experiment, using a CO-laser pump and a diode-laser probe. The CO laser, operating on the 13-12 P(15) line at 1775.2588 cm(-1), is locked by Lamb-dip stabilization to one of the Stark components of the a(R)R(9, 9) nu(4) line of NH(3). The diode laser probes the aQ(9, 9) nu(2) line at 921.255 cm(-1), revealing a complex spectrum of sub-Doppler features, the narrowest of which are 5.3 +/- 0.3 MHz wide. As well as the resonances associated with population depletion of the common lower levels, we see line-narrowing effects that are due to two-quantum Raman-type processes and collision-induced resonances arising from state-changing collisions that preserve the molecular velocities. The zero-field a(R)R(9, 9) nu(4) line is established to be 264.1 +/- 5.0 MHz above the CO-laser line.

FIR Stark spectroscopy on optically pumped CH3OH laser

An extensive theoretical and experimental analysis of the absorption and emission spectrum of a CH3OH FIR-laser excited by a conventional CO2 laser is presented. Particular interest is devoted to the Stark shifts of the pump and lasing lines and to the electric field dependence of the Fir-laser output of the various lines. The offsets with respect to the exciting radiation and the Stark shifts of the IR absorption (pump) lines are measured by means of the transferred Lamb dip technique. The theoretical behaviours of the Stark patterns are calculated for several choices of the quantum numbers and selection rules involved in the transitions. A large variety of experimental results are reported and compared to theory. Non-linear Stark shifts have been observed for the 37.5μm FIR laser line and for the IR-pump transitions excited by the 9-P(38) and 10-R(38) CO2 laser Lines. Line assignments are proposed and new FIR laser lines are reported.

CO_2 laser coincidences with ν_3 of SiF_4 near 97 μm

Doppler-limited tunable-diode laser spectra of the stretching fundamental (ν3) of 28SiF4 at 1031 cm−1 have been analyzed and the spectroscopic constants determined. Explicit identifications have been made for transitions near CO2 laser lines between 1023 and 1038 cm−1; 51 such transitions have been observed in sub-Doppler saturation spectra obtained with a CO2 laser. The closest observed coincidence is R(53) F19 of 28SiF4, at −1.391 MHz from 12C16O2P(30). Implications for isotope-enrichment experiments are discussed.

Continuous-wave Stark far-infrared lasing lines in ^15NH_3 optically pumped by a CO_2 or N_2O laser

We report preliminary results of a systematic research of far-infrared (FIR) emissions in (15)NH(3) by Stark-tuning the infrared-pumped transitions to the CO(2) or N(2)O laser lines; 28 new FIR lasing lines have been detected for static-electric fields up to 62 kV/cm.

Hyperfine and Λ-doubling parameters for the v = 1 state of NO from infrared-radiofrequency double resonance

Hyperfine Λ-doubling transitions have been measured for a number of rotational levels with 2.5 ≤ J ≤ 20.5 in the 2 Π 1 2 v = 0 and 1 states of 14N 16O. The measurements were made with infrared-radiofrequency double-resonance spectroscopy, utilizing close coincidences between CO laser lines and NO 1-0 band transitions in the 5.4-μm region. A combined fit of the present and previous data gives new information for NO in the v = 1 state: greatly improved Λ-doubling parameters and the first determination of hyperfine interaction parameters.

High-resolution spectroscopy of matrix-isolated ReO_4^− molecules

The temperature-dependent linewidth of the saturable infrared active nu(3) vibrational mode (10.8 microm) of ReO(4)(-) molecules in pure-KI host crystals has been measured between 2 and 600 K. Below 20 K the absorption line is inhomogeneously broadened and has a 0.016-cm(-1) width (FWHM). Above 30 K the transition is homogeneously broadened by elastic acoustic phonon-scattering processes. The low-temperature inhomogeneous width of ReO(4)(-) molecules in KI:RbI alloys is 2 orders of magnitude larger than for the pure KI crystal, and the absorption is coincident with the 10.8-microm P(42) CO(2) laser line at all temperatures.

The Q-branch rotational spectrum in the ground state of CD4, studied by laser–rf double resonance spectroscopy

Near coincidences between CO2 and N2O laser lines and transitions in the n2 and n4 infrared bands of CD4 have been investigated using laser–rf double resonance spectroscopy. Six rf transitions have been assigned to ground state DJ = 0 rotational transitions of CD4, with J values of 7, 7, 10, 19, 19, and 21. A least squares fit of the observations using quartic and sextic tensor distortion constants only yields the parameters Dt = 32649.7±1.7 Hz, H4t = −1.9747±0.0044 Hz, and H6t = 1.1027±0.0016 Hz.

Stark modulation of CO2 laser in a Lorentz broadened molecular gas

The Stark modulation of a CO2 laser line in a Lorentz-broadened gas is quantitatively investigated and some features (like the change of modulation sign with pressure) are explained. Considering symmetrical top molecules, a simple formula is derived for the Stark modulation which describes its dependence on molecular parameters, gas pressure and electric field, in agreement with the experiment.

A monochromatic and frequency stabilized cw source in the 5 μm region obtained by doubling the frequencies of a cw laser in tellurium

In this paper we describe the construction of a monochromatic and frequency stabilized continuous wave (cw) source in the 5 μm region. It is built up by a cw single line, single-mode frequency and amplitude stabilized CO2 laser. This primary CO2 laser source is then frequency-doubled in a tellurium crystal using second harmonic generation (SHG). The obtained emission is monochromatic (half-width 50 kHz). The frequency can be varied discreetly from 4.6 μm to 5.5 μ depending on the CO2 laser line chosen.