Ionization of two-photon excited states $$5p^{\mathrm {5}}(^{\mathrm {2}}P_{\mathrm {3/2}})6\hbox {p}[^{\mathrm {3}}/_{\mathrm {2}}$$ , $$^{\mathrm {5}}/_{\mathrm {2}}]_{\mathrm {2}}$$ , $$\hbox {M}=2$$ (jl-coupling) of xenon atoms by circularly polarized probe light was studied experimentally in a supersonic beam. The observed photoionization signals revealed oscillation structure due to the Larmor precession of atomic states in an external magnetic field. We derived analytical formulas for the photoelectron current and explained the diversity in the structure of the detected oscillations in terms of the principal lines among multiplet components of optical transitions. The obtained numerical data demonstrate collapse and revival (beating) behavior of the photocurrent due to nonlinearity of Zeeman shifts in the presence of the Paschen–Back effect. Our results indicate the possibility of implementing Doppler-free spectroscopy involving bound-free transitions.

Исследована ионизация когерентно двухфотонно возбужденной суперпозиции 4f-состояний атома Xe фемтосекундным импульсом пробного лазера в сверхзвуковом пучке с регулируемой задержкой между импульсами накачки и пробным. Регистрируемая колебательная структура в сигналах фотоионизации связана с когерентными биениями суперпозиции возбужденных состояний. Высокая эффективность предложенной схемы регистрации квантовых биений обусловлена практически 100%-й эффективностью сбора фотоэлектронов в отличие от ранее используемой регистрации флуоресценции. Ключевые слова: квантовые биения, двухфотонное возбуждение, фемтосекундная ионизация.

Ionization of coherently two-photon-excited superposition of 4 f states of Xe in a supersonic atomic beam is studied by means of femtosecond pump–probe spectroscopy. Oscillatory structure in the photoionization signal is related to coherent beats of a superposition of excited states. High efficiency of the proposed scheme of registration of quantum beats was achieved due to nearly 100% efficiency of collecting photoelectrons, in contrast to detection of fluorescence used traditionally.

The excitation spectrum of the resonance luminescence of Na atoms in a mixture with CF4 exhibits a satellite transition corresponding to the simultaneous optical excitation of the colliding atom and molecule, Na(2S1/2) + CF4( $${{v}_{3}}$$ = 0) + h $$v$$ → Na(2P1/2, 3/2) + CF4( $${{v}_{3}}$$  = 1), where $${{v}_{3}}$$ is the infrared active mode of CF4 with a vibrational quantum energy of 1283 cm–1. It is shown that the optical coupling between the upper and lower states of this asymptotically $$({{R}_{{{\text{Na}} - {\text{C}}{{{\text{F}}}_{4}}}}} \to \infty )$$ forbidden transition can be explained within a model that takes into account the polarization of the atom in the field of the molecule and the interaction of the dipole moment of the CF4( $${{v}_{3}}$$  = 1 ↔  $${{v}_{3}}$$  = 0) transition with the dipole moments of electronic transitions in the atom. The results of calculations based on this model are in satisfactory agreement with the experiment. It is noted that CF4 may be of interest as a component of a working medium of diode pumped alkali lasers.

Ionization of polarized states of Ar and Xe atoms by femtosecond probe pulse in a supersonic beam in the presence of magnetic field was studied theoretically and experimentally. The revealed oscillation structure in the photoionization signals occurs due to the Larmor precession of exited atomic states in a magnetic field. We derived analytical formulas for the photoelectron current and explained the detected oscillations in terms of photon and atomic polarization moments. Our results indicate the possibility of implementing Doppler-free spectroscopy involving bound-free transitions.

The absorption and luminescence excitation spectra of gas phase mixtures of alkali metals atoms A (A = K, Rb, Cs) with carbon tetrafluoride molecules CF4 are studied in the region of transitions from the ground state A(S-2(1/2)) to the lowest resonance states A(P-2(1/2,3/2)). It is shown that these transitions have intense satellites corresponding to the simultaneous vibrational excitation of the carbon tetrafluoride molecule, A(S-2(1/2)) + CF4(v(3) =0) + hv -> A(P-2(1/2,3/2)) + CF4(v(3)=1), where v(3) is the IR active mode of CF4 with a vibrational quantum energy of 1281 cm(-1). The satellites are relatively narrow bands (FWHM 30 cm(-1)) slightly (< 10 cm(-1)) shifted to the red from the energies corresponding to the asymptotes A(P-2(J)) + CF4 (v(3)=1), J=1/2, 3/2. The spectral width and position of the satellites indicate that these transitions occur at relatively large atom-molecule distances. In contrast to the majority of the studied before pair excitation processes in the gas phase, the intensity borrowing mechanism responsible for the observed satellite bands cannot be explained by the dipole-dipole interaction. Possible alternative mechanisms are discussed. It is also established that mixtures with CF4 are chemically stable including at elevated temperatures up to at least 200 degrees C and their optical excitation results in resonance luminescence of alkali atoms. As compared to Ar, collisions with CF4 provides a higher rate of the population transfer between the A(P-2(1/2)) and A(P-2(3/2)) states. These results indicate that CF4 may be used as a component of the active medium of diode pumped alkali lasers. (C) 2020 Elsevier Ltd. All rights reserved.

The ionization processes of the ArM, XeN and XeNArM clusters in a supersonic beam with multiphoton excitation were studied. The third 263 nm harmonics of a femtosecond Ti : sapphire laser were used for excitation and ionization. Kinetic energy spectra of photoelectrons has been recorded by a magnetic bottle time-of-flight electron spectrometer. Analysis of the electronic spectra of multiphoton ionization of clusters shows that, in heteronuclear clusters, the transfer of excitation energy between xenon and argon atoms plays a significant role in ionization.

Measurements of the mass distribution of xenon clusters in a pulsed supersonic beam during ionization by electrons and photons are carried out. For ionization by electron impact, an electron gun was used. The energy of the electron beam varied in the range 15 – 70 eV. Third harmonic of a titanium-sapphire femtosecond laser with a wavelength of 263 nm was used for multiphoton optical ionization. Clusters of xenon XeN with N < 1000 were observed in a supersonic beam,. It is shown that the shape of the mass spectrum in multiphoton optical and electronic ionizations coincide at the electron energy near the ionization threshold (15 – 20 eV). At the electron energy of 30 – 70 eV the shape of the mass spectrum is substantially distorted in the region N ∼ 2 – 150. The change in the shape of the mass spectrum is mainly due to the large fragmentation of multiply ionized xenon clusters due to Coulomb decay.

Впервые показано, что при изменении временных параметров возбуждающего импульса лазера (величина чирпа, длительность импульса и его амплитуда) можно управлять величиной и знаком эффекта " конденсации спектра" импульса на частоте резонансного перехода плотной резонансной среды.Важно отметить, что

For the first time, it is demonstrated that the magnitude and sign of the effect of spectral condensation of a laser pulse at the resonant-transition frequency of a dense medium can be controlled by changing the driving-pulse parameters (chirp, pulse width, and pulse amplitude). In the process of this, importantly, the driving-pulse energy and spectrum remain unchanged. Direct time-resolved measurements revealed an oscillatory character of the induced superradiance of rubidium vapors representing a long train of decaying short pulses. The width and repetition rate of the pulses in the train are determined by atomic density N-0 of the medium, while the width of an entire superradiance pulse (10 ps) is considerably larger than that of the driving laser pulse (50 fs).

It is known that ultrashort pulse coherent propagation through the resonant absorbing medium leads to the pulse splitting and pulse train generation. First of these pulses is the input pulse and the following pulses are resonant medium response. It is also known that such a propagation mode leads to the "spectrum condensation" effect, at which broad-band input femtosecond laser emission, as it leaves opically-dense medium, centralized in the vicinity of the resonance absorption lines. It's interesting to clear up the dependence of a condensation effect on an input femtosecond pulse phase modulation value (chirp).

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.

The main achievements in the analytical application of lasers, their potential applications, and prospects for further development of this field were described. The role of laser generation conditions and modes, especially of pulse duration, on laser applicability in analysis was discussed. The propagation of powerful ultrashort laser pulses (USPs) in transparent samples and the positive role of chirping and filamentation were considered. New methods of laser-induced breakdown spectroscopy based on ultrashort pulses, chirping, and filamentation were discussed.

We experimentally studied the superradiance of the resonant line of rubidium under femtosecond optical pumping of a dense extended medium without population inversion under conditions of strong lightmatter coupling. Substantial self-splitting of superradiance components is observed.

In this paper, we investigated the coherent degenerate and nondegenerate four-wave interaction in the propagation of a femtosecond laser pulse in atomic rubidium vapor. In the case of degenerate four-wave interaction, the mechanism for optical switching in an optically dense resonant medium is considered. The third-order nonlinear interaction of the laser beams intersecting in the cell at a small angle leads to a nonlinear response in the form of a fourth beam, which occurs as a result of the interaction between the two pump beams and the third signal laser beam.