P. R. Berman

Theory of collision effects on atomic and molecular line shapes

A review of recent developments in the theory of the effects of binary collisions on the spectral profiles associated with atomic and molecular systems is presented. To consistently account for collisional perturbations of both the internal energy levels and the velocity of active (emitting or absorbing) atoms or molecules, one must use a theory in which the center-of-mass motion of the active atoms has been quantized. Following this procedure general equations for absorption or emission line shapes are obtained. The line shapes may exhibit narrowing or broadening with increasing perturber pressure, depending upon the nature of the collision interaction. The physical significance of the collision mechanisms giving rise to such behavior is discussed, as is the experimental evidence in support of the theory. Various applications of the theory are presented.

Effects of collisions on linear and non-linear spectroscopic line shapes

Abstract A fundamental physical problem is the determination of atom-atom, atom-molecule and molecule-molecule differential and total scattering cross sections. In this work, a technique for studying atomic and molecular collisions using spectroscopic line shape analysis is discussed. Collisions occuring within an atomic or molecular sample influence the samples absorptive or emissive properties. Consequently the line shapes associated with the linear or non-linear absorption of external fields by an atomic system reflect the collisional processes occuring in the gas. Explicit line shape expressions are derived characterizing linear or saturated absorption by two- or three-level “active” atoms which are undergoing collisions with perturber atoms. The line shapes may be broadened, shifted, narrowed, or distorted as a result of collisions which may be “phase-interrupting” or “velocity-changing” in nature. Systematic line shape studies can be used to obtain information on both the differential and total active atom-perturber scattering cross sections.

Heating or cooling collisionally aided fluorescence

Abstract Collisionally aided redistribution of scattered laser light is suggested as a method to cool or heat gaseous samples. The efficiency is evaluated and restricting conditions are considered. Some potential applications are given.

Study of Collisions by Laser Spectroscopy

Publisher Summary This chapter discusses the use of laser spectroscopy for probing collision effects in atomic and molecular systems. The development of tunable narrow-band laser sources has led to a number of opportunities for spectroscopic collision studies using “bulb” experiments. Using near-resonant monochromatic excitation, atoms with a specified velocity component in the direction of the fields propagation vector are excited. The excited atoms can then undergo collisions in which their velocities are altered and the velocity changes can be monitored by subjecting the atoms to a probe field. In this manner, information on the differential scattering cross section can be obtained from the laser spectroscopy. With the use of lasers, non-thermal excited-state populations can now be achieved. Bulb experiments require less excited-state population than beam experiments because photon detection efficiency is generally greater than neutral-particle detection efficiency. Bulb experiments are quite flexible in nature, containing components that are easily interfaced with other experiments. Moreover, the laser spectroscopic techniques can also enable studying inelastic processes such as magnetic reorientation, fine structure changing, and rotational reorientation collisions.

Theory of pump-probe spectroscopy

We calculate the two-level pump–probe absorption coefficient including both upper-to-lower-level decay and level decays to a still lower-lying reservoir level. The probe-absorption profile can have arbitrary ratios of the natural linewidth and detuning to the Doppler width. We observe and explain new line-shape features that occur when the two main levels decay to the reservoir level at different rates.

Laser-induced collisional energy transfer

Abstract A review of the status of theoretical and experimental work on Laser-Induced Collisional Energy Transfer (LICET) is presented. The process involves two dissimilar atoms, one of which is excited, colliding in the presence of a laser field. If the radiation field is properly tuned to an interatomic resonance, the initially excited system undergoes a transition to its ground state, while its partner gains both the excitation energy and that of a photon. The transfer cannot occur unless both collisional and radiative interactions are present. The line shape of the excitation spectrum, markedly asymmetric, is closedly related to the interaction dynamics. Details of theories applicable to low intensities, as well as their high-field counterparts are discussed, as are the predictions of spectral shapes and comparison with measurements. The treatment is extended to include studies of magnetic-state and electronic-state coherences, allowing an additional insight into the nature of the process. Representative experimental investigations are surveyed, including a detailed description of experimental arrangements specifically designed for high-accuracy spectral measurements. Finally, it is shown how a wider approach to the LICET process, by considering it as a radiative transition of a transient molecule, can be promising for an improved understanding of the problem.

Theory of photon echoes in dressed atoms

Abstract A theory is presented which describes the formation of photon echoes in an atomic vapor when a cw laser field (the “dressing” field) drives an atomic transition while a sequence of pulses is applied to a coupled transition. It is found that, with a two-pulse excitation scheme, as many as nine echoes may be produced. The relationship of these “dressed-atom” echoes to conventional photon echoes is discussed.

Photon echoes using double-resonance optical pulses

A theory of photon echoes using double-resonance optical pulses (PEDROP) is presented. Two double-resonance pulses, separated by a time interval T, are used to cause transitions among three states of an atom. Each double-resonance pulse consists of two simultaneously applied laser pulses that drive coupled transitions in the atom. It is shown that for time t > T, six photon echoes are produced (three on each of the coupled transitions) at five distinct times. Moreover, a nonradiating macroscopic coherence is also produced between states of the same parity. With the use of an interrogation pulse to monitor this coherence, three additional photon echoes can be produced on either of the coupled transitions. A Doppler phase diagram is constructed that enables one easily to predict the times and conditions of echo formation. PEDROP combines features of both two-photon and trilevel echoes but offers advantages over these methods for studying relaxation phenomena.

Optical coherent transients induced by time-delayed fluctuating pulses

A theory is developed to describe the optical transient signals that arise when a sample of two-level atoms is irradiated by a sequence of two or three broad-bandwidth pulses is presented. The first two pulses have a relative delay time of order of the correlation time of the pulse fluctuations. These pulses whose temporal width is much greater than the delay time are fully correlated with one another and can be strong enough to saturate the two-level atomic transition. In the case of three-pulse transients the third pulse is weak non-correlated with the first two and is delayed in time so that it does not overlap them. Taking into account the effects ofinhomogeneous and homogeneous broadening we calculate the intensity of the transient signals as a function of delay time. The signals are found to depend dramatically on the intensities of the excitation pulses. It is shown that for strong excitation pulses a direct dependence of the signal on the cross-correlation time of the pulses r occurs that does not exist when the pulses are weak. In particular for saturating pulses the signals exhibit a peak of width of order r . In the case of the two-pulse transient the peak is found to disappear when the Doppler width of the atomic ensemble becomes sufficiently large. This peak can have a very narrow dip near its maximum whose width is much

Noble Gas Induced Relaxation of the Li 3S-3P Transition Spanning the Short Term Impact Regime to the Long Term Asymptotic Regime

Photon echoes have a Doppler free character which allows one to study relaxation processes which would otherwise be hidden in the inhomogeniously broadened spectral profile. It has recently been shown, for example, that contrary to expection, a radiating atom in a linear superposition of dissimilar electronic states can undergo identifiable velocity changing collisions [1]. Studies of this nature require an examination of the sub-Doppler region of the spectral line shape. The effect manifests itself, in the case of photon echoes, in a dependence of the effective relaxation cross section σeff on the excitation pulse separation τ. In this paper we report measurements in Li vapor where τ can be increased into the regime where σeff once again becomes independent of τ. In the limit τ=0 we measure σ0 which is the phase changing cross section as calculated by Baranger [2] while in the large τ limit we measure σ∞ the average total scattering cross section of the ground and the excited states. Our data at intermediate values of τ is used to determine the form of the scattering kernel and the average velocity change per collision. These measurements are for the 2S–2P superposition states in atomic Li perturbed by each of the noble gases. For He perturbers the scattering kernel is found to be Lorentzian, for the other perturbers it is Gaussian.