Phase conjugation of CO2-laser radiation by a three-beam interaction

The distortion correction of a CO₂-laser radiation wavefront is studied by using forward resonant parametric scattering in gaseous SF₆ and retrace after retroreflection from a mirror.     

Circular dichroism in K-shell ionization from fixed-in-space CO and N2 molecules

We have measured the angular distributions of 1s photoelectrons excited by circularly and linearly polarized light from fixed-in-space CO and N2 molecules, in the vicinity of their shape resonances. A strong circular dichroism, i.e., a strong dependence on the sense of rotation of the polarization vector of the photons, is found for both molecules. State-of-the-art one-electron multiple scattering and partially correlated random phase approximation calculations are in good agreement with many, but not all, aspects of the experimental data.     

Stepwise photoionization of complex organic molecules in the gas phase induced by UV laser radiation

We measured the photoionization cross-sections of anthracene, 1,2-benzanthracene,p-terphenyl, and 2,5-diphenylfuran molecules excited to the singlet and triplet electronic states intermediate in the stepwise two-photon ionization process at 266 nm (all compounds) and at 355, 391 and 417 nm (anthracene).     

Collisionless excitation of NH3 molecules via one- and two-photon transitions by multimode radiation of TEA CO2-laser

The excitation of ammonia by one- and two-photon transitions by TEA CO₂-laser radiation has been studied under collisionless conditions. It has been shown that the experimental results of two-photon excitation of ¹⁴NH₃ molecules in the 2v ₂ a(1, 1) a(1, 1) transition by the 10P(24) CO₂-laser line, the frequency offset of which is 0.02cm^–1, can be described well by modelling the laser radiation as a broadband chaotic field. The experimental results of one-photon excitation in ¹⁴NH₃ and ¹⁵NH₃ [4], with the offset values exceeding the CO₂-laser linewidth, can be also described well with the use of a chaotic field with a continuous spectrum. In the case of a nearly exact one-photon resonance, however, the excitation efficiency of molecules depends critically on the real mode structure of the CO₂-laser radiation.     

Electrical and optical characteristics of a multichannel N2-laser

The influence of electrical parameters on the optical properties of atmospheric pressure transversely excited N₂-lasers of the Blumlein-type is investigated. Experiments are performed using a multichannel laser system providing high repetition rate trains of nanosecond pulses (50 to 100 MHz). Both the high voltage rise and decay times of the order of several kV/ns are determined electro-optically and correlated with the intensity and time lag of the optical pulses. Using an injection technique the intensity and beam divergence could be markedly improved.     

Multiphoton fragmentation and ionization of CF2HCl molecules and clusters by UV radiation

The results of experimental studies of multiphoton ionization of CF₂HCl molecules and clusters by UV laser radiation in the wavelength range 217–236 nm are reported. In the case of molecules, the main reaction products are CF₂H⁺ and CF⁺ ions as well as atomic chlorine. It is found that the spectra of the products of ionization of free molecules and molecules condensed into clusters differ qualitatively: multiphoton ionization of clusters does not yield CF₂H⁺ ions. The dependences of the ion yield on the intensity of laser radiation and its wavelength are measured. The effect of a constant electric field and the radiation spectral width on the multiphoton ionization process is demonstrated. The shape of the velocity distributions is determined for a number of products. A strong anisotropy is detected in the reaction of formation of CF₂H⁺ ions. Possible mechanisms for these processes are discussed.     

Gas breakdown near solid targets by pulsed CO2-laser radiation

Conclusions In conclusion, let us note some important problems whose solution, in our opinion, will make for a better undertanding of the physical mechanisms of processes occuring in a low-threshold breakdown of gases near solid targets. First is the systematization of data on threshold-breakdown intensities (dependence of S_br on the area of the irradiation spot and laser-pulse duration over wide ranges of these quantities, on the pressure and kind of gas surrounding the target, and on the radiation wavelength). Analysis of such data would make it possible to construct a more orderly theory of the process. Also of great interest is the transfer of heat to a solid target from the plasma of the low-threshold breakdown of a gas and from the shock waves propagating in the plasma. It is advisable to carry out numerical calculations on the gasdynamic processes occurring near a solid target under irradiation by a pulsed CO₂ laser, with account for the counterpressure of the ambient gas and for various regimes breakdown-plasma propagation with subsequent comparison of these data with experimental results.Translated from Izvestiya Vysshykh Uchebnykh Zavedenii, Fizika, No. 11, pp. 34–60, November, 1977.     

Single and multiple ionization of N2 molecules by intense light fields of femtosecond duration

We report studies on the multiple ionization of the N₂ molecule using intense, femtosecond laser pulses. We present details of the experimental characterisation of the light pulses and analysis and detection of the ions produced. Precautions to be taken in intense field ionization experiments are discussed. We illustrate the retrieval of information about different aspects of the ionization process (such as, kinetic energies of the fragments produced, dissociation bond lengths and information on the precursors to fragmentation) using coincidence techniques. We report results of the first measurements of the angular distribution of a highly charged fragment, N³⁺.     

Self-diffraction of CO2-laser radiation in SF6

Self-diffraction of CO₂-laser radiation (=10.51 m) has been observed in gaseous SF₆. Different orders of self-diffraction have been studied, up to the third. The amplification due to degenerate resonant multiphoton parametric processes has been measured to be as large as eight. The experimental estimates have been derived for the effective nonlinear susceptibilities (coefficients of nonlinear interaction) of the third, fifth, and seventh orders. The self-diffraction may essentially affect nonlinear processes including optical phase conjugation.     

Structural transformations of polysaccharides exposed to CO2-laser radiation

We have used IR spectroscopy to study conformational transformations of polysaccharide macromolecules (dextran, pullulan, amylose, and microcrystalline cellulose) and changes in their structural ordering when exposed to cw emission of a CO₂ laser. We have established that the structures of conformationally labile polysaccharides are the most sensitive to laser radiation. The effect of laser treatment is greatest for lower molecular weight polymers. We have observed for the first time changes in the conformational states of dextran macromolecules in the direction of an increase in the structural ordering when exposed to emission from a CO₂ laser. This quite different from purely thermal treatment, where disordering of the polysaccharides structure occurs. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 2, pp. 164–169, March–April, 2006.     

Effective upconversion of CO2-laser radiation in sodium vapors

The effective upconversion of CO₂-laser radiation to the uv spectral range has been obtained in sodium vapors, the photon efficiency being 65%, the corresponding power efficiency being 2640%.     

Effects of a spark switch and stray capacities on the operation of a N2-laser

The laser-tube stray capacity and the time dependent spark resistances of the pulsing system influence the operation of a N₂-laser. Their analysis with experimental and analog-computer methods yields an optimum value of the stray capacity. In the case of a 40 kV pulse from a 2-gap Marx generator with a 0.4 J energy input we obtain 120 kW laser output with 2.5 ns halfwidth at an optimum stray capacity of 120 pF. A substantially reduced threshold value and increased laser output were observed with a fast single-gap transmission line pulser of the “Nanolite” type.     

The absorption of pulsed CO2-laser radiation by ethylene at total pressures from 25 to 3000 Torr

The absorption of pulsed CO₂-laser radiation by ethylene has been measured at total pressures from 25 to 3000 Torr, using the P(12) and P(14) lines in the 10.6 m band, with incident fluences from 0.1 to 0.7 J/cm². Marked deviations from the Beer-Lambert absorption law were observed, with the effective absorption coefficient varying with pressure, fluence, absorption path-length and the addition of non-absorbing gas. Pressure broadening of the rotational lines of the ethylene absorption spectrum was shown to be the major cause of these deviations, together with lesser effects which can be attributed to the rise in temperature of the absorbing gas during the laser pulse.     

A high power N2-laser of long pulse duration

An N₂-laser of 19 ns pulse-duration and an energy of 30 mJ has been developed and examined. A dc preionization, a grooved electrode and a partial electrical screening of the discharge tube permit reproducible single-shot operation and yield a homogeneous beam.     

Nonlinear ionization of organic molecules in high intensity laser fields

We use a series of 23 organic molecules to study ionization of complex media caused by their interaction with intense 40 fs, 0.8 &mgr;m pulses. All molecules reach saturated ionization at higher intensities than would be expected for atoms of the same ionization potential, reminiscent to what has been reported for dielectric breakdown with femtosecond pulses. Dependence of the ionization rate on the alignment of the molecule with the laser field is ruled out as the cause of the high saturation intensities. All molecules allow a significant range of intensities between the region of approximately 100% ionization and before the second and subsequent electrons are removed.     

Collisionless multiphoton ionization of water molecules by a TEACO2 laser

Collisionless ionization of water molecules into fragments of H⁺, H₂⁺, O⁺, OH⁺, H₂O⁺ were observed using a high power TEACO₂ laser. The mechanism could be related to that of selective dissociation of complex molecules by the same laser.