Vibrational properties of single-wall BC3 nanotubes

The phonon dispersions of single-wall BC₃ nanotubes with any chirality are calculated within a symmetry-based force constant model of the lattice dynamics. Based on the non-symmorphic symmetry group of the BC₃ tubes, the symmetries and number of the Raman- and infrared-active modes at Γ point of the one-dimensional Brillouin zone are given. The neighbor atom-atomic interaction force constants are recalculated by fitting them to the experimental phonon energy-dispersion curves of honeycomb BC₃ sheet. The frequencies of the optically active modes are presented as the function of diameters and chiralities for BC₃ tubes. The obtained phonon density-of-states spectra, phonon dispersion relations, and vibrational patterns of the zone-center phonons are presented and discussed in detail. The calculated frequencies of infrared-active modes are compared with the experimental values reported in the literature. The results provide comprehensive information about the vibrational properties of the BC₃ tubes and shed light on the interpretation of Raman scattering and infrared spectroscopies.     

Dynamics of photoprocesses induced by femtosecond infrared radiation in free molecules and clusters of iron pentacarbonyl

The dynamics of photoprocesses induced by femtosecond infrared radiation in free Fe(CO)₅ molecules and their clusters owing to the resonant excitation of vibrations of CO bonds in the 5-μm range has been studied. The technique of infrared excitation and photoionization probing (λ = 400 nm) by femtosecond pulses has been used in combination with time-of-flight mass spectrometry. It has been found that an infrared pulse selectively excites vibrations of CO bonds in free molecules, which results in a decrease in the yield of the Fe(CO)₅⁺ molecular ion. Subsequent relaxation processes have been analyzed and the results have been interpreted. The time of the energy transfer from excited vibrations to other vibrations of the molecule owing to intramolecular relaxation has been measured. The dynamics of dissociation of [Fe(CO)₅]_n clusters irradiated by femtosecond infrared radiation has been studied. The time dependence of the yield of free molecules has been measured under different infrared laser excitation conditions. We have proposed a model that well describes the results of the experiment and makes it possible, in particular, to calculate the profile of variation of the temperature of clusters within the “evaporation ensemble” concept. The intramolecular and intracluster vibrational relaxation rates in [Fe(CO)₅]_n clusters have been estimated.     

Vibrational wave packet dynamics in the silver tetramer probed by NeNePo femtosecond pump-probe spectroscopy

We present results on the ultrafast dynamics of mass-selected neutral Ag₄ clusters using NeNePo (negative ion - neutral - positive ion) femtosecond pump-probe spectroscopy. One-color pump-probe spectra of the Ag₄ ^-/Ag₄/Ag₄ ⁺ system measured at 385 nm and an internal cluster temperature of 20 K display a complex beat structure over more than 60 ps. The oscillatory structure is attributed to vibrational wave packet dynamics in an excited “dark" state of neutral Ag₄. A dominant 740 fs wave packet period as well as wave packet dephasing and rephasing are observed in the spectra. Fourier analysis of the spectra yields a group of frequencies centered around 45 cm^-1 and an anharmonicity χ _e2νχ _eχ _e of 2.65 cm^-1 for the active vibrational mode. Received 30 November 2000     

Lattice dynamics of ZnSe x S1 − x semiconductor crystals

Recently ultrabroadband infrared solid state lasers based on a new vibronic material Cr²⁺:ZnSe _x S_1–x were demonstrated [1–3]. Cr²⁺ ion substitutes the metal ion (tetrahedral sites), the crystal field of the solid solution is responsible for large inhomogeneous broadening of Cr²⁺ electron states. The crystal field can be reconstructed by investigation of lattice dynamics — optical phonon parameters and dielectric function in IR. We paid special attention to investigation of vibrational and infrared spectroscopic properties of ZnSe _x S_{1 ? x} crystals. A very interesting and somewhat unexpected result of these studies was the existence in the crystals of effective S-Se dipoles, which generate an additional deep dynamically charged level in the forbidden gap of the semiconductors. The results of the first-principles calculations of both the phonon structure and the electron localization in ZnSe _x S_1–x crystals as well as acceptor levels in Cr²⁺: ZnSe crystal are discussed.     

Femtosecond laser embedded grating micromachining of flexible PDMS plates

We report on the femtosecond laser micromachining of photo-induced embedded diffraction grating in flexible Poly (Dimethly Siloxane) (PDMS) plates using a high-intensity femtosecond (130 fs) Ti: sapphire laser (λ_p = 800 nm). The refractive index modifications with diameters ranging from 2 μm to 5 μm were photo-induced after the irradiation with peak intensities of more than 1 × 10¹¹ W/cm². The graded refractive index profile was fabricated to be a symmetric around from the center of the point at which femtosecond laser was focused. The maximum refractive index change (Δn) was estimated to be 2 × 10⁻³. By the X-Y-Z scanning of sample, the embedded diffraction grating in PDMS plate was fabricated successfully using a femtosecond laser.     

Vibrational spectra of ternary ordered vacancy compounds

We have studied the lattice dynamics of A²B ₂ ³ C ₄ ⁵ crystals with the thiogallate structure using a nonpolarized ion model. We computed the interatomic interaction parameters for nine compounds. The calculated phonon spectra, density of vibrational states, and temperature dependence of the specific heats agree well with the available experimental data. V. D. Kuznetsov Siberian Physicotechnical Institute, Tomsk University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 85–89, February, 1998.     

Influence of vapor transport equilibration on Raman spectra of Er:LiNbO3 crystals

Raman spectra of as-grown and vapor transport equilibration (VTE) treated Er:LiNbO₃ crystals, which have different cut orientations (X-cut and Z-cut), different Er-doping levels (Er:(0.2, 0.4 and 2.0 mol%)LiNbO₃) and different VTE durations (80, 120, 150 and 180 h), were recorded at room temperature in the wavenumber range 50-1000 cm⁻¹ by using backward scattering geometry. The spectra were attributed on the basis of their spectral features and the previous experimental work and the most recent theoretical progress in lattice dynamics on pure LiNbO₃. In comparison with the pure crystal the most remarkable effect of Er-doping on the Raman spectrum is observed for the E(TO₉) mode. It does not appear at 610 cm⁻¹ as the pure crystal, but locates at 633 cm⁻¹. In addition, the doping also results in the lowering of the Raman phonon frequency, the broadening of the Raman linewidth and the changes of the relative Raman intensity of some peaks. The VTE treatment results in the narrowing of the linewidth, the recovery of the lowered phonon frequency and the further changes of relative Raman intensity. The narrowing of Raman linewidth indicates that the VTE processing has brought these crystals closer to stoichiometric composition. The VTE treatment has induced the formation of a precipitate ErNbO₄ in the high-doped Er(2.0%):LiNbO₃ crystals whether X- or Z-cut. For these precipitated crystals, besides above linewidth and phonon frequency features, they also display more significant Raman intensity changes compared with those not precipitated crystals. In addition, a slight mixing between A₁(TO) and E(TO) spectra is also observed for these precipitated crystals. Above doping and VTE effects on Raman spectra were quantitatively or qualitatively correlated with the characteristics of the crystal structure and phonon vibrational system.     

Study of lattice vibrational modes in CuGaS2 by using fourier transform spectroscopy

The lattice dynamics of a single crystal of CuGaS₂, grown by iodine transport technique, have been studied by using far IR absorption spectroscopy. All the absorption maxima caused by the phonon excitation are compared with the lattice vibrational modes obtained by Raman spectroscopy and by IR reflection techniques. An absorption maximum located at 175 cm^?1 cannot be explained with the help of phonon excitation; however this peak can be attributed to the defect frequency originating from the replacement of gallium atom by sulphur in the v₁₇ mode of vibration. The frequency of this defect-induced vibrational mode is calculated by taking a modified molecular model approach, and is found to be 166.9 cm^?1, which is in reasonably good agreement with the experimentally observed value of 175 cm^?1.     

Probing solvation dynamics with femtosecond vibrational spectroscopy

We explain why solvent reorganization can induce both red- and blue-shifting of vibrational transitions of a particular probe molecule upon excitation to the S₁ electronic state. We observe with femtosecond vibrational spectroscopy, after hydrogen-bond cleavage dynamics, an additional blue shift of the carbonyl stretch of coumarin 102 of 7 cm^-1 when dissolved in chloroform. Received: 28 June 2000 / Published online: 7 August 2000     

Possibilities for controlling attosecond x-ray pulse generation during molecular ionization by femtosecond laser radiation

We discuss the role of different factors (molecular sizes and configuration, orientation of the molecular axis with respect to the electric field of a laser pulse, the type of the molecular orbital, etc.), which characterize molecules and their state, in the formation of the nonlinear response of a molecule ionized by a high-power femtosecond laser pulse. In numerical experiments within the framework of a two-dimensional model for the H ₂ ⁺ molecular ion, we study possibilities for controlling the process of nonlinear frequency conversion of femtosecond optical radiation into X-ray radiation of attosecond duration by means of preliminary vibrational or electronic excitation of molecules. We demonstrate the possibilities of using the attosecond pulse generation as a diagnostic tool for probing vibration-rotational dynamics of molecules.     

Laser crystallization of amorphous silicon films investigated by Raman spectroscopy and atomic force microscopy

The intrinsic and phosphorous (P)-doped hydrogenated amorphous silicon thin films were crystallized by laser annealing. The structural properties during crystallization process can be investigated. Observed redshifts of the Si Raman transverse optical phonon peak indicate tensile stress present in the films and become intense with the effect of doping, which can be relieved in P-doped films by introducing buffer layer structures. Based on experimental results, the established correlation between the stress and crystalline fraction (X_C) suggests that the relatively high stress can limit the increase in X_C and the highest crystalline fraction is obtained by a considerable stress release. At high laser energy density of 1250 mJ/cm², the poorer crystalline quality and disordered structure of the film originating from the irradiation damage and defects lead to the low electron mobility.     

Femtosecond dynamics of primary processes in visual pigment rhodopsin

The dynamics of the coherent photoisomerization of the 11-cis-retinal in bovine rhodopsin is studied by femtosecond time-resolved laser absorption spectroscopy with a resolution of 30 fs. Rhodopsin is excited with 500-, 535-, and 560-nm femtosecond pulses to produce different initial Franck-Condon states with different vibrational energies of the molecule in its electronically excited state. The time evolution of the photoinduced differential absorption spectra of rhodopsin is measured upon excitation with a femtosecond pulse in a spectral range from 400 to 720 nm. Oscillations in the time-resolved absorption of the rhodopsin photoproducts, such as photorhodopsin with a vibrationally excited all-trans-retinal and in its initial-state rhodopsin with a vibrationally excited 11-cis-retinal, are examined. It is demonstrated that these oscillations reflect the dynamics of coherent vibrational wavepackets. The Fourier transform of these oscillatory components yields frequencies, amplitudes, and initial phases of various vibrational modes involved in the motion the wavepackets in both photoproducts. The main vibrational modes manifest themselves at frequencies of 62 and 160 cm^?1 for photorhodopsin and 44 and 142 cm^?1 for initial-state rhodopsin. It is shown that these vibrational modes are directly involved in the coherent reaction under the study, with their amplitudes in the power spectrum produced by the Fourier transform of the kinetic curves being dependent on the wavelength of rhodopsin excitation.     

The influence of the anion vibrational temperature on the fs dynamics in a NeNePo experiment

A negative-neutral-positive (NeNePo) femtosecond charge-reversal experiment studying the temperature-dependent relaxation dynamics of linear Ag₃ is described. A wavepacket is prepared on the potential-energy surface of the electronic ground state of the neutral trimer by photodetachment from an anion ensemble held at different, well-defined temperatures between 20 K and 350 K. The wavepacket dynamics is probed by resonant two-photon ionization. The cooled octopole ion trap developed to prepare the cold anions is described. The relaxation dynamics of the initially linear Ag₃ from a saddle point of the potential-energy surface into the triangular equilibrium configuration shows a significant dependence on the anion temperature, which is rationalized in terms of a simple model. We demonstrate that a low anion vibrational temperature results in the generation of “narrower” wavepackets on the neutral potential-energy surface. This allows us to probe coherent effects more sensitively. Received: 10 January 2000 / Published online: 24 July 2000     

Investigation of weak damage in Al0.25Ga0.75As/GaAs by using RBS/C and Raman spectroscopy

The weak damage induced by 0.8 MeV Si ion implantation in the Al_0.25Ga_0.75As films epitaxially grown on GaAs substrates was studied by using Rutherford backscattering spectrometry/channeling (RBS/C) and Raman spectroscopy. RBS/C spectra measured from the implanted samples showed rather low damage level induced by the ion implantation with ion dose from 1×10¹⁴ to 5×10¹⁵ cm⁻². The Raman spectra were measured on these samples. Two kinds of phonon modes, GaAs-like and AlAs-like, are observed, which indicate the existence of multiple phonon vibrational modes in the epitaxial Al_0.25Ga_0.75As films on the GaAs substrate. Compared with the unimplanted sample, the Raman photon peaks for the implanted sample shift gradually to lower energy with the increase of the implantation dose. The strains induced in the implanted layer were also evaluated from the Raman spectra. The result from high resolution double crystal X-ray diffractometry (HRXRD) also verified the evolution of the strains in the implanted layers.     

Photoluminescence of hexagonal boron nitride: Effect of surface oxidation under UV-laser irradiation

We report on the UV laser-induced fluorescence of hexagonal boron nitride (h-BN) following nanosecond laser irradiation under vacuum and in different environments of nitrogen gas and ambient air. The observed fluorescence bands are tentatively ascribed to impurity and mono (V_N) or multiple (m-V_N with m=2 or 3) nitrogen vacancies. A structured fluorescence band between 300 and 350 nm is assigned to impurity-band transition and its complex lineshape is attributed to phonon replicas. An additional band at 340 nm, assigned to V_N vacancies on surface, is observed under vacuum and quenched by adsorbed molecular oxygen. UV-irradiation of h-BN under vacuum results in a broad asymmetric fluorescence at ∼400 nm assigned to m-V_N vacancies; further irradiation breaks more B-N bonds enriching the surface with elemental boron. However, no boron deposit appears under irradiation of samples in ambient atmosphere. This effect is explained by oxygen healing of radiation-induced surface defects. Formation of the oxide layer prevents B-N dissociation and preserves the bulk sample stoichiometry.     

Photodissociation dynamics of CF3I investigated by two-color femtosecond laser pulses

We report an experimental study of the multiphoton dissociation dynamics of CF₃I performed on a home-built femtosecond laser pump-probe system, with time-of-flight mass spectrometer. The first repulsive A band and the 5pπ³7sσ υ₂=1 Rydberg state of CF₃I were accessed by one- and two-photon transitions at 267 nm, respectively, with the latter two-photon absorption followed by a further two-photon probe transition at 401 nm to the state of the parent ion. The observed signals from the CF₃ ⁺ and I⁺ fragments show similar multi-component exponential decay patterns but the former is 4 times stronger than the latter. However, the parent CF₃I⁺ signal was observed to evolve in a very different manner, decreasing sharply when probed in the first 289 fs following excitation and subsequently rising again after 860 fs to a constant level below that measured at negative pump-probe delay times when the pump and probe pulses exchange roles. This dip observed in the parent ion profile, is very different from that previously reported at shorter pump wavelengths of 264 nm or 265 nm, and is interpreted as the competition between two different ionization channels. One from the vibrationally excited υ₂=1 of the irradiated Rydberg state and the other from the dissociative vibrational origin of the same electronic state which is populated by internal vibrational relaxation.     

Raman spectra of the orthorhombic semiconductor compound Cu2SnTe3

The optical phonon spectrum of the semiconductor Cu₂SnTe₃, that crystallizes in the orthorhombic structure with space group Imm2 (), have been studied by measuring unpolarized Raman scattering between 10 and 300 K. The experimental frequencies of the phonon modes observed were compared to those calculated by using simplified lattice dynamical models reported in the literature. From combined analysis of these results together with the factor group analysis of the zone-center vibrational modes, valuable information about these modes was obtained and their possible symmetry was assigned. A₁ modes at 71, 123, 167, 176 and 190 cm⁻¹; A₂ modes 115 and 131 cm⁻¹; B₁ modes at 76, 142 and 152 cm⁻¹; B₂ modes at 89, 100 and 206 cm⁻¹; a overtone at 246 cm⁻¹, and combinations at 218, 270 and 292 cm⁻¹; have been observed in this compound.     

Molecular dynamics calculations of anharmonic properties of the vibrational spectrum of BCC zirconium under pressure

The structural stability and lattice dynamics of the high-pressure bcc phase of Zr at a constant temperature T = 500 K are studied for various volumes using molecular dynamics simulation with the Animalu pair pseudopotential. Dispersion curves of the vibrational spectrum calculated by the molecular dynamics method for various volumes are compared to the phonon spectrum obtained in the harmonic approximation. It is demonstrated that, as the volume decreases, all frequencies of the vibrational spectrum increase gradually and bcc zirconium remains strongly anharmonic along all high-symmetry directions of the Brillouin zone over the entire range of volumes studied. The strongly anharmonic N _T1 phonon is significantly softened near the point of structural instability of bcc-Zr at T = 500 K and V = 0.87V ₀. As the volume decreases to V = 0.73V ₀ under pressure, the anharmonic corrections for this phonon decrease by almost an order of magnitude and the phonons near the H point of the Brillouin zone become anharmonic. The damping of the T ₁ phonon mode along the [110] direction is calculated as a function of pressure.     

Argon ions induced thermoluminescence properties of Ba0.12Sr0.88SO4 phosphor

Thermoluminescence properties of barium strontium mixed sulfate have been studied by irradiation with Argon ions. The sample was recrystallized by chemical co-precipitation techniques using H₂SO₄. The X-ray diffraction study of prepared sample suggests the orthorhombic structure with average grain size of 60 nm. The samples were irradiated with 1.2 MeV Argon ions at fluences varying between 10¹¹ and 10¹⁵ ions/cm². The argon ions penetrate to the depth of 1.89 μm and lose their energy mainly via electronic stopping. Due to ion irradiation, a large number of defects in the sample are formed. Thermally stimulated luminescence (TSL) glow curves of ion irradiated Ba_0.12Sr_0.88SO₄ phosphor exhibit broad peak with maximum intensity at 495 K composed of four overlapping peaks. This indicates that different sets of traps are being activated within the particular temperature range each with its own value of activation energy (E) and frequency factor (s). Thermoluminescence (TL) glow curves were recorded for each of the ion fluences. A linear increase in intensity of TL glow peaks was found with the increase in ion dose from 59 kGy to 5.9 MGy. The kinetic parameters associated with the prominent glow peaks were calculated using glow curve deconvolution (GCD), different glow curve shape and sample heating rate methods.     

Control of wavepacket dynamics in mixed alkali metal clusters by optimally shaped fs pulses

We have performed adaptive feedback optimization of phase-shaped femtosecond laser pulses to control the wavepacket dynamics of small mixed alkali-metal clusters. An optimization algorithm based on Evolutionary Strategies was used to maximize the ion intensities. The optimized pulses for NaK and Na₂K converged to pulse trains consisting of numerous peaks. The timing of the elements of the pulse trains corresponds to integer and half integer numbers of the vibrational periods of the molecules, reflecting the wavepacket dynamics in their excited states. Received 4 December 2001