Vibrational-rotational behavior of diatomic heteronuclear molecular systems in intense laser pulses

The dynamics of diatomic heteronuclear molecules in the low-frequency intense laser pulses is studied by numerical solution of the time-dependent Schr?dinger equation and both rotational and vibrational degrees of freedom are taken into account. The interference stabilization against the dissociation process is found to take place in a strong field and is shown to result in trapping of population in different rotational-vibrational bound states due to efficient Raman V- and ??-type transitions. The interplay between the vibrational and rotational dynamics of a molecule is investigated. The dissociation suppression due to the interference mechanism and all its attributes are established in the case of multiphoton coupling between the initial state and dissociation continuum.     

Stabilization of a Rydberg atom and competition between the Λ and V transition channels

The photoionization and stabilization of a Rydberg atom in a strong laser field are investigated theoretically. The role of Raman-type transitions between neighboring Rydberg levels via the continuum (Λ transitions) and via lower-energy resonant Rydberg levels (V transitions) is analyzed. The conditions under which this phenomenon can be observed experimentally are determined. The characteristics of stabilization due to V-type transitions are described. Zh. éksp. Teor. Fiz. 114, 821–836 (September 1998)     

Strong field effects in the system of two interacting Rydberg atoms

The ionization dynamics of two interacting Rydberg atoms in a strong laser field has been investigated. Each atom has been described in the “two discrete levels + continuum” model. Quasienergy states in this system, which describe field-dressed atoms, have been studied. It has been shown that one of the quasienergy states corresponds to the formation of an atomic state stable against ionization, which leads to the interference stabilization regime first observed in the case of individual atoms in [M. V. Fedorov and A. M. Movsesian, J. Phys. B 21, L155 (1988)]. Methods for creating entangled states in this system have been proposed and the dynamics of entanglement in the process of interaction with the laser field has been analyzed.     

Rotation and alignment of diatomic molecules and their molecular ions in strong laser fields

A theory of classical rotation and alignment of diatomic molecules with and without permanent dipole moments and of their molecular ions in strong laser fields is developed. The actions of both cw and pulsed laser fields is considered. Conditions under which molecular axes are aligned with the field, which is presumed to be linearly polarized, have been determined. The analysis leads to a conclusion that molecules exposed to ultrashort laser pulses continue to rotate even after the end of the laser pulse. The effect of dynamical chaos on the rotational angular velocity in strong laser field is discussed. Zh. éksp. Teor. Fiz. 113, 573–582 (February 1998)     

The D<Superscript>1</Superscript>Π state of the NaRb molecule

We present a detailed experimental study of the D¹Π state of the NaRb molecule by means of Fourier transform spectroscopy of laser induced fluorescence. The entire data field for the D¹Π state of Na⁸⁵Rb and Na⁸⁷Rb consists of rovibrational levels with v^′=0-39 and J^′=1-200. The data were incorporated into a direct fit of a single potential energy curve to the level energies using the Inverted Perturbation Approach method. The D¹Π state q factors, which describe the Λ-doubling, have been obtained in a wide range of rotational and vibrational quantum numbers. Analysis revealed several perturbation regions in the D¹Π state. Electronic supplementary material Online Material     

Role of weak transitions in multiphoton excitation of molecules by IR laser radiation

Simultaneous excitation of a considerable part of molecules from many rotational levels of the ground state to higher vibrational states by IR laser radiation can be explained by considering weak transitions in a rotational band structure as it is shown at the example of SF₆ molecule. Very accurate compensation of anharmonicity in relatively wide spectral interval at comparatively low intensity of laser radiation can be explained on this basis. The considered scheme can be applied to the molecules of various symmetry with arbitrary anharmonicity.     

Features of the photon statistics of two coupled electromagnetic field modes under the conditions of strong mode coupling

The probabilities of transitions between the Fock states of two electromagnetic field modes under the influence of coupling between modes of finite duration are investigated. It is shown that the transition probability is a strongly oscillating function of the mode numbers of the photons. Under conditions in which the coupling frequency exceeds the geometric mean of the mode frequencies (strong coupling), large numbers of photons are excited in the mode with the lower frequency. The excitation of a two-dimensional radiation field oscillator and the “red” asymmetry of the transition probabilities can be attributed to instability of the classical two-dimensional oscillator with strong mode coupling. Zh. éksp. Teor. Fiz. 112, 128–136 (July 1997)     

Exciton absorption and optical-gain bands in the presence of laser radiation

The absorption and gain bands of a weak probe signal in the presence of the Bose-Einstein condensation of excitons, which appears under nonequilibrium conditions in a field of coherent laser radiation, are considered. It is shown that the absorption of light is caused by a quantum transition from the ground state of the crystal to the quasiexciton branch of the spectrum. Amplification of the signal occurs as a result of transitions from the quasiexciton branch of the spectrum to the ground state of the crystal. Fiz. Tverd. Tela (St. Petersburg) 40, 924–927 (May 1998)     

Determination of the helium-4 mass in a Penning trap

The determination of the rotational quadrupole alignment of diatomic molecules via REMPI detection is investigated. In this process a high focal intensity usually increases the detection probability. At high intensities the AC Stark effect may cause a splitting of the normally degenerate m_J sublevels of a rotational state J beyond the spectral width of the exciting radiation. This leads to a selective detection of only certain m_J states with the consequence that deduced alignment factors can be misleading. From the theoretical considerations line profiles are explicitly calculated for dynamic polarizabilities which represent the B ¹Σ⁺ _u←X ¹Σ⁺ _g transition of H₂, in order to fit an experimental (3+1) REMPI spectrum and to predict (1+1') line shapes as a function of laser intensity. It is further shown that the deduced quadrupole alignment factor A ₀ ⁽²⁾ is significantly changed by the second order AC Stark effect when the intensities are chosen high enough to observe asymmetric broadened line profiles. Different combinations of relative linear polarizations of the exciting and ionizing laser beams are discussed. Received 1st August 2000 and Received in final form 2 May 2001     

Ultrafast vibronic phase transitions induced in semiconductors by femtosecond laser pulses

The intermode anharmonic interaction in the theory of ultrafast (t∼10⁻¹³ s) vibronic phase transitions induced on semiconductor surfaces (Si, GaAs) by femtosecond laser pulses is calculated. The conditions for plasma-induced transitions either to a state of chaotic disorder in the positions of the atoms (“cold liquid”) or into a state with crystal symmetry different from the initial symmetry (a new crystalline phase) are determined. It is shown that a NaCl-type structure is realized in GaAs for a transition of the second type, the transition being due to the instability of the longitudinal optical phonon branch. The corresponding numerical estimates are made for Si and GaAs. Fiz. Tverd. Tela (St. Petersburg) 41, 1462–1466 (August 1999)     

Dynamics of molecular alignment steered by a few-cycle terahertz laser pulse

The field-free alignment of molecule ClCN is investigated by using a terahertz few-cycle pulse (THz FCP) based on the time-dependent density matrix theory. It is shown that a high degree of molecular alignment can be obtained by changing the matching number of the THz FCPs in the adiabatic regime and the non-adiabatic regime. The matching number can affect both the maximum value of the alignment and the time at which it is achieved. It is also found that a higher degree of alignment can be achieved by using the THz FCP at lower intensity and there exists an optimal threshold of molecular alignment with the increase of the field amplitude. Also found is the frequency sensitive region in which the degree of maximum alignment can be enhanced greatly by modulating the center frequencies of different THz FCPs. The investigation demonstrates that comparing with a THz single-cycle pulse, a better result of the field-free alignment can be created by a THz FCP at a constant rotational temperature of molecule.     

多原子分子在强红外激光场作用下的多光子光致离解理论

本文从非线性振动理论和单分子化学反应的观点出发,提出一个物理模型用以解释红外激光场作用下的多原子分子瞬时离解现象。我们指出:实验上通常观察到的离解阈值现象与非线性振动中固有的“跳变”过程有着密切的内在联系,以SF₆分子为例,计算并分析了它的离解阈值、频率“红移”和转动补偿等过程,得到在定量上大致与实验现象相符的结果。 关键词：     

Selective excitation of the molecular rotational wave packet by two time-delayed laser pulses

In this paper, we investigate the control of the molecular wave packet of a linear molecule by two femtosecond laser pulses. It is shown that the odd and the even rotational wave packets created by a single laser pulse can be selectively excited by accurately controlling the time delay of another laser pulse. By inserting the peak of the second laser pulse at the position of maximum or minimum value around quarter or three quarter rotational period of the slope curve with odd (or even) rotational wave packet contribution that is created by the first laser pulse, the odd rotational wave packet can be enhanced (or suppressed) while the even rotational wave packet is suppressed (or enhanced). As a result, the molecular alignments around quarter and three quarter rotational periods can be obtained. Moreover, it is also shown that by inserting the second laser pulse around the quarter or three quarter rotational periods, the changes in the maximum degree of the molecular alignment for the odd and the even rotational wave packet contributions are consistent with their corresponding slope curves at these positions.     

Charged mobile complexes in magnetic fields: A novel selection rule for magnetooptical transitions

The implications of magnetic translations for internal optical transitions of charged mobile electron-hole (e-h) complexes and ions in a uniform magnetic field B are discussed. It is shown that transitions of such complexes are governed by a novel exact selection rule. Internal intraband transitions of two-dimensional (2D) charged excitons X ⁻ in strong magnetic fields are considered as an illustrative example. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 8, 504–509 (25 October 1999) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Pulsed Doppler-free two-photon spectroscopy of polyatomic molecules

Doppler-free two-photon electronic spectra of a large polyatomic molecule are recorded for the first time with pulsed laser radiation of near Fourier-transform limited bandwidth (Δv～100 MHz). The resolution obtained is sufficient to resolve individual rotational lines. Due to the high density of these rotational transitions a strong Doppler-broadened background is observed, which is, however, subtantially reduced by suitable choice of photon polarizations. Different vibronic bands of benzene (C₆H₆) are investigated and very accurate rotational constants are found.     

Effect of laser intensity and of lower-state rotational energy transfer upon temperature measurements made with laser-induced fluorescence

4 –air flame, with OH at 2000 K. We calculate the ratio of LIF intensities that would be induced by doubled dye-laser light near 283 nm, by means of the A←X, 1←0, P₁(7), and Q₂(11) transitions in OH. Here we show that the ratio of LIF signals from those two transitions, and thus the deduced temperature, is sensitive to laser intensity. That is caused mainly by the competition between laser-pumping of molecules out of the lower rotational state and of rotational energy transfer (RET) collisions into that state. A-state collisional effects are normally important, but are minimized here by assuming that they are the same for both transitions. The laser spectral intensity dependence of the fluorescence ratio depends heavily upon the value of the RET coefficients within the X-state. While RET reduces the sensitivity of the observed signal to the laser spectral intensity, the conversion of a measured fluorescence ratio to temperature is particularly difficult. That is because RET rates, and quenching rates, can be a function of local conditions and of the rotational state being populated. Two different models are used to demonstrate these effects, and both predict large effects upon temperature. Received: 19 February 1998/Revised version: 16 June 1998     

Field dependence of Young’s modulus in a gadolinium single crystal

The field dependence of Young’s modulus along the hexagonal c axis is measured in a gadolinium single crystal over a wide range of temperatures and magnitudes of the magnetic field aligned with the c axis. It is found that the isotherms of the field dependence of the ΔE effect in gadolinium are well approximated by a linear dependence on the square of the magnetization in both strong and weak strong magnetic fields, and also above and below the spin reorientation temperature. It is shown that the experimental trends obtained near the ferromagnetic transition can be interpreted within the approach based on the Landau theory of second-order phase transitions. The parameters of such an approach are determined for gadolinium on the basis of the experimental data. Zh. éksp. Teor. Fiz. 114, 2111–2121 (December 1998)     

On the fine structure of the rotational bands of superdeformed nuclei

It is shown that the staggering of the energy levels of the superdeformed rotational bands can be explained from first principles, using the standard microscopic Hamiltonian with the pairing, quadrupole, and hexadecapole residual interactions, as resulting from rotation-induced nonadiabatic distortion of the mean field in the nucleus. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 4, 231–236 (25 August 1996)     

Orientational states of C60 molecule in crystals

Local symmetry of orientational states of the C₆₀ molecule in crystals has been investigated. It was shown that the various orientational phase transitions in different crystals are related to different orientational orbits. The model of orientational phase transitions based on a sequence of orientational states with different symmetry properties has been suggested. We have found that both the local symmetry of C₆₀ molecule and the symmetry of its internal vibrations become higher after a reduction of crystal spatial symmetry at the phase transition. This effect is fairly common and can be observed in the orientational order-disorder phase transitions with wave vectors at the Brillouin zone boundary. Feasible manifestations of the predicted effect in various experiments are discussed. Zh. éksp. Teor. Fiz. 113, 1081–1093 (March 1998)     

Creation and manipulation of coherences in molecules with a pulsed magnetic field

We demonstrate the application of a pulsed magnetic field for the creation and manipulation of coherences in molecular systems, using quantum beat spectroscopy for the detection of the dynamics of the molecular superposition states. In all cases, the experiments are performed on energy levels in electronically excited states of the (jet-cooled) CS₂ molecule populated by a short laser pulse. In the basic experiment, following excitation of initially degenerate Zeeman sublevels under zero field conditions with suitable laser polarization, quantum beats are generated at the moment the magnetic field is switched on, even when the field is delayed by several excited state lifetimes. By quenching of the field, it is shown that the molecule may be “frozen” in any superposition state of the participating sublevels. Using a combination of static and pulsed fields with different orientations, the molecule can be prepared in a more general state, described by coherences among all Zeeman substrates. This is achieved by choosing an appropriate time delay for the switched field, without any change to the geometrical parameters of the experiment such as laser polarization or detection direction. Numerical simulations of these dynamical coherence phenomena have been performed to support assignment and interpretation of the experimental results. Received: 8 April 1998 / Accepted: 3 June 1998