A. A. Pastor

Experimental Evaluation of the Carrier Lifetime in GaAs Grown at Low Temperature

The relaxation dynamics of nonequilibrium charge carriers in gallium arsenide epitaxial films grown by molecular-beam epitaxy at low temperatures has been studied. The growth conditions of the epitaxial layer provided an excess arsenic content of 1.2% in the layer. In a material of this kind, the carrier lifetime is <1 ps. To examine carrier relaxation in the femtosecond range, an original scheme for measuring the refractive index dynamics was developed on the basis of the pump-probe technique. The lifetime of nonequilibrium charge carriers was evaluated to be (200 ± 35) fs.

Study of the lowest electronic states of Xe 2 , XeKr, and XeAr molecules by the method of multiphoton resonance ionization

The spectra of Xe2, XeKr, and XeAr molecules in the range 66 500–68 800 cm−1 are obtained by the methods of (2 + n) and (3 + n) (n = 1, 2, 3) resonance multiphoton ionization during the registration of molecular and atomic ions. The combining of two-and three-photon resonance excitations of Xe2 molecules makes it possible to obtain the spectra caused by transitions from the ground state X0g+ to the excited states of Xe*6s[3/2]1,2oXe1S0 molecules both of the even (0g+, 1g) and of the odd (B0u+, B′1u, 2u) symmetries. The data on the Ω = 2u state of the Xe2 molecule with the dissociation limit Xe*6s[3/2]2o + Xe1S0 and on the Ω = 1 state of the XeAr molecule with the dissociation limit Xe*6s[3/2]1o + Ar1S0 are obtained for the first time. The potential curve of the excited 2u state of the Xe*6s[3/2]2o Xe1S0 molecule is repulsive and intersects the potential curve of the B0u+ state of the Xe*6s[3/2]1o Xe1S0 molecule. In the case of the three-photon excitation, it is observed that all the bands in the spectra of XeKr and XeAr molecules are broadened and are shifted, which indicates that, in an intense light field, the influence of the dynamic Stark effect is significant.

Effect of annealing on the nonequilibrium carrier lifetime in GaAs grown at low temperatures

GaAs samples grown by molecular-beam epitaxy at low (230°C) temperatures are investigated. One of the samples is subjected to aftergrowth annealing at 600°C. Using an unconventional pump-probe scheme for measuring the dynamic variation in the light refractive index, the nonequilibrium charge-carrier lifetime (275 ± 30 fs before annealing) is determined. Such a short carrier lifetime in the unannealed material is due to the high concentration of point defects, mainly AsGa antisite defects. According to X-ray diffraction and steady-state optical absorption data, the AsGa concentration in the samples is 3 × 1019 cm−3, which corresponds to an arsenic excess of 0.26 at %. Upon annealing at 600°C, the superstoichiometric As defects self-organize and form As nanoinclusions in the GaAs crystal matrix. It is shown that in this case the nonequilibrium charge-carrier lifetime increases to 452 ± 5 fs. This lifetime is apparently ensured by the capture of non-equilibrium charge carriers at metal As nanoinclusions.

Multiphoton mass spectra of XeKr molecules in the range of excited Xe*6p[5/2](2,3) atoms

Data on excited states of XeKr molecules in the energy range 78280–77600 cm−1 are obtained. Using the method of multiphoton laser photoionization of molecules in a supersonic jet, five vibrational progressions of XeKr molecules are obtained, which are attributed to five electronic-vibrational transitions from the ground state of the XeKr molecule of the symmetry 0+ to excited states of the symmetry Ω = 0+, 1, 2 with the dissociation limit Kr1S0 + Xe*6p[5/2]2 and of the symmetry Ω = 1, 2 with the dissociation limit Kr + Xe*6 p [5/2]3. The molecular constants of the corresponding excited states of the XeKr molecule are estimated.

Electronic spectra of XeNe molecules in the range 77100-90100 cm-1 near Xe* (6p, 5d, 6p’, 7s, 7p, 6d) obtained by the (3 + 1) REMPI and (2 + 1) REMPI methods

The electronic spectra of XeNe molecules in the range of 77100-90100 cm-1 are measured by the method of laser resonance multiphoton ionization in a supersonic jet. The photoionization spectra are obtained upon two- and three-photon excitations of molecules and their ionization by the next photon. In the range of 80300-90100 cm-1 near Xe*(5d, 6p’, 6d, 7s, and 7p), the spectra are obtained for the first time. A whole number of vibrational systems are measured in this range. The majority of vibrational systems near Xe* (5d, 6d, 7p, and 7s) are located in the red range with respect to their dissociation limits. In the blue range with respect to the dissociation limits, continua corresponding to transitions of molecules from the ground state to repulsive potential curves of excited states are detected. For a number of excited states of XeNe molecules, the vibrational analysis is performed and molecular constants are estimated.

The study of ungerade electronic states of the Xe2 molecules in the region of Xe*(5p56p, 5d, 7s, 6d) by the resonance multiphoton ionization method

Electronic spectra of the Xe2 molecules in the energy range of 77700–89300 cm−1 are recorded. The method of resonance enhanced multiphoton ionization of molecules in a supersonic molecular beam was used, in which excitation of the molecules by three photons was followed by ionization caused by a fourth photon (the (3+1) REMPI method). Analysis of the vibrational structure of observed systems of bands yielded information about the dissociation energy and the molecular constants for ungerade states of molecules. On the basis of the Franck-Condon principle, the equilibrium distances for potential curves were estimated from the relative intensities in vibrational progressions. Data on 16 new electronic states of diatomic xenon molecules with the dissociation limits Xe2* → XE(5p61S0) + Xe*(5p56p,5d, 7s, 7p) were obtained.

Discharge plasma in an argon–water vapor mixture as a gas laser medium

The possibility of obtaining the inverse population of the excited states of hydroxyl molecules generated in low-pressure discharge plasma in an Ar–H2O mixture was studied. The possibility of inversion of both the A2Σ+ state relative to the X2Π ground state and inside the vibrational-rotational structure of the levels of the A2Σ+ excited state of the OH molecule was analyzed.

Relaxation channels of multi-photon excited xenon clusters

The relaxation processes of the xenon clusters subjected to multi-photon excitation by laser radiation with quantum energies significantly lower than the thresholds of excitation of atoms and ionization of clusters were studied. Results obtained by means of the photoelectron spectroscopy method showed that desorption processes of excited atoms play a significant role in the decay of two-photon excited xenon clusters. A number of excited states of xenon atoms formed during this process were discovered and identified.

Measuring femtosecond lifetimes of free charge carriers in gallium arsenide

Peculiarities of the measurement of femtosecond lifetimes of free charge carriers in gallium arsenide grown by the method of molecular beam epitaxy are considered. The carrier lifetime is determined by a variant of the pump-probe method in which the pumping and probing radiation beams are incident onto the semiconductor sample surface at the Brewster angle. The proposed variant ensures the detection of a change in the induced refractive index even in the case of very small response signals, thus providing for a high sensitivity of measurements. It is shown that the relative signal changes near the Brewster angle reach a maximum level.

Relaxation times measurement in single and multiply excited xenon clusters

Direct measurement of the rates of nonradiative relaxation processes in electronically excited xenon clusters was carried out. The clusters were created in a pulsed supersonic beam and two-photon excited by femtosecond laser pulses with a wavelength of 263 nm. The measurements were performed using the pump-probe method and electron spectroscopy. It is shown that relaxation of light clusters XeN (N < 15) predominantly occurs by desorption of excited xenon atoms with a characteristic time constant of 3 ps. Heavier electronically excited clusters (N > 10) vibrationally relax to the lowest electronically excited state at a rate of about 0.075 eV/ps. Multiply excited clusters are deactivated via energy exchange between excited centers with the ionization of one of them. The production of electrons in this process occurs with a delay of ∼4 ps from the pump pulse, and the process is completed in 10 ps.