V. S. Letokhov

TIME-RESOLVED LASER OPTOACOUSTIC TOMOGRAPHY OF INHOMOGENEOUS MEDIA

The methods of time-resolved laser optoacoustic tomography of inhomogeneous media and related problems are reviewed. Time-resolved laser optoacoustic tomography allows one to measure the distribution of light absorption in turbid media with depth resolution up to several microns in real time. The theory of laser excitation of acoustic waves by absorbing of light in particles, dispersed in transparent, light-absorbing or scattering media, is developed. The distribution of light absorption can be obtained from the temporal course of acoustic pressure. Two schemes of acoustic wave detection — in the medium under testing (direct detection) and in transparent medium, coupled to the investigated one (indirect detection) — are discussed. In both cases the reconstruction of light absorption can be made by simple calculations. Test experiments with homogeneous and layered media confirm the proposed theoretical models and the possibility of using the proposed experimental schemes. Light absorption in homogeneous, inhomogeneous media and in absorbing particles dispersed in turbid media was investigated. The experimental setup allows one to measure the absorption coefficients over the range 1-500 cm−1 with the depth resolution 10–15 μm over the depth 1–1.5 mm.

Electromagnetic trapping of cold atoms

This review describes the methods of trapping cold atoms in electromagnetic fields and in the combined electromagnetic and gravity fields. We discuss first the basic types of the dipole radiation forces used for cooling and trapping atoms in the laser fields. We outline next the fundamentals of the laser cooling of atoms and classify the temperature limits for basic laser cooling processes. The main body of the review is devoted to discussion of atom traps based on the dipole radiation forces, dipole magnetic forces, combined dipole radiation–magnetic forces, and the forces combined of the dipole radiation–magnetic and gravity forces. Physical fundamentals of atom traps operating as waveguides and cavities for cold atoms are also considered. The review ends with the applications of cold and trapped atoms in atomic, molecular and optical physics.

Selective dissociation of SF6 molecules in a two-frequency infrared laser field

Abstract The dissociation of SF 6 in the field of two infrared frequencies is studied. It is shown that the selectivity of dissociation is increased compared to the single frequency dissociation case. The dispersion characteristics of the vibrational quasicontinuum have been obtained. A theoretical model of the processes is discussed.

Stepwise laser photoionization of molecules in a mass spectrometer: a new method for probing and detection of polyatomic molecules

Laser spectroscopy and kinetic mass spectrometry of polyatomic isolated molecules have been carried out by using stepwise laser ionization of molecules in a mass spectrometer. The optical absorption spectrum of NO(2) has been recorded, for example, by using pulsed dye-laser excitation and H(2) vacuum-ultraviolet laser ionization of molecules. The kinetics of excited electron states and of the production of molecular and fragmented ions of some polyatomic molecules have been investigated with high temporal resolution. The sensitivity achieved is 10(-10) cm(-1) of the absorption coefficient and 10(9) (10(5)) molecules in the ground (electronically excited) state in the irradiated volume.

The visible luminescence kinetics of BCl3 in the field of a high-power CO2 Laser

Abstract The visible luminescence in BCl 3 in the field of a high-power short-pulse CO 2 laser is studied. It is shown that there are two time regions for the luminescence: the instantaneous, which arises simultaneously with the laser pulse, and a delayed luminescence, which is pressure dependent. Possible mechanisms which cause both types of luminescence are discussed.

Photolysis of molecules by an infrared high-power laser pulse

Abstract We report the spectra of visible luminescence which accompanies the infrared photolysis of molecules by an intense CO 2 laser pulse. The molecules studied were C 2 H 4 , C 2 F 3 Cl, CH 3 OH, CH 3 NO 2 , CH 3 CN, CF 2 Cl 2 and BCl 3 . A gated system recorded the signal during the first 100–200 ns after the laser pulse, i.e., during the collisionless dissociation phase. The dissociated products were identified. The mechanism of the infrared photolyses is discussed and compared with UV photolyses.

Cooling and trapping of atoms and molecules by a resonant laser field

Abstract A method is suggested for simultaneous cooling and trapping of atoms and molecules in a low-pressure gas under forces caused by recoil during spontaneous or induced transitions of the particles in the resonance field of a three-dimensional standing light wave. It is shown that at light field intensities ∼0.01–0.1 W/cm2 it is possible to cool atoms and molecules to the resonance field photon momentum and to hold the particles in the light field volume during a long period of time. The proposed approach opens the way for high-resolution Doppler-free spectroscopy of a small number of atoms and molecules.

Narrow resonances of saturated absorption of the asymmetrical molecule CHFCiBr and the possibility of weak current detection in molecular physics

Abstract According to V.S. Letokhovs suggestion the neutral and charged weak currents (in weak interactions) must cause a splitting of the vibrational-rotational levels of left and right molecules by a relative value of 10 -15 –10 -16 . To detect this effect an experiment is proposed which consists of direct measuring the beat frequency of two frequency stabilized lasers, their references being two identical narrow spectral lines in the vibrational-rotational spectrum of two optical isomers of an asymmetrical molecule. For this purpose one may use narrow saturated absorption resonances of CHFClBr induced in the cw CO 2 laser field. The effects having influence on the reproducibility of the frequency by molecular reference and the necessity of identity conditions for cells with left and right molecules (CHFClBr) are discussed.

Optoacoustic detection of multiple photon molecular absorption in a strong IR field

Abstract A new method based on the optoacoustic effect has been proposed to measure multiple photon absorption at vibrational molecular transitions in a strong IR laser field. Comparison measurements of the average absorbed energy have been done by this method for molecules with different dissociation limits in a strong CO 2 -laser field, the field intensity changed therewith by four orders. For the poly-atomic molecules C 2 H 4 and SF 6 having a comparatively low dissociation limit, the absorption increases monotonously as the power density of laser radiation P increases up to the dissociation limit. For three-atom molecules, such as D 2 O and OCS, absorption saturation takes place with P ≳ 10 MW/cm 2 .

Laser guiding of atoms in a hollow optical fiber

Abstract The laser guiding of atoms in a hollow optical fiber is discussed. Our estimates show that for reasonable laser power 1W, the use of atomic guidance in an optical waveguide allows an atomic flow to be transmitted without any substantial loss over a distance of about ten meters.