The pulsed Li atomic beam produced in our experiment is based on controlled transversely-excited-atmospheric
CO2 laser--induced ablation of a Li metal target. The atomic beam is propagated in vacuum or in
CO2 gas at low pressure. Atoms in the beam are probed by laser-induced fluorescence spectroscopy. This allows the determination of time-of-flight and velocity distributions. Li Rydberg states (n=5--13) are populated in the beam by two-step pulsed-laser excitation. The excited atoms interact with
CO2 molecules. l- and n-changing cross sections are deduced from the time evolution of the resonant or collision-induced fluorescence following this selective excitation. l-changing cross sections of the order of
104 A
˚2 are measured; they increase with n as opposed to the plateau observed for
Li∗ colliding with a diatomic molecule. This behavior is qualitatively well explained in the framework of the free-electron model. n\ensuremath{\rightarrow}n' changing processes with large cross sections (10--100 A
˚2) are also observed even in the case of large electronic energy change (\ensuremath{\Delta}
Enn′>103 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$). These results can be interpreted in terms of resonant-electronic to vibrational energy transfers between Li Rydberg states and
CO2 vibrational modes.
The distribution of principal quantum numbers for high-Rydberg states of lithium, produced in a thermal atomic beam by electron impact excitation, has been determined after passage of the beam through gaseous targets. Target densities were selected for an approximately tenfold reduction of the on-axis flux of Rydberg atoms. The state distribution, determined by electric field ionization, is markedly affected by a target of rapidly rotating polar molecules having rotational energy level separations comparable to the binding energy of the Rydberg valence electron. However, the distribution is not appreciably influenced by rare-gas or slowly rotating polar molecular targets, for which the Rydberg beam spreads as a result of momentum transfer.
l-mixing collisions Li(
n2D)+X\ensuremath{\rightarrow}Li(
n2L,L>2)+X induced in Rydberg states of lithium (n=4,5,...,13) by rare-gas atoms or Li ground-state atoms are studied in a heat-pipe arrangement. The l-mixing cross sections deduced from the time analysis of the
n2D laser-induced fluorescence agree with experimental values obtained with the heavier alkali-metal atoms and with different theoretical models developed for quasielastic Rydberg-electron--target collisions. Quenching of the
n2D fluorescence at higher rare-gas pressures is interpreted as inelastic collisional transfer of the L\ensuremath{\ge}1 mixed states to the nonhydrogenic S sublevel. Finally the radiative lifetimes of the
n2D states and
n2L mixed states with L\ensuremath{\ge}1 are deduced, respectively, from the low-pressure limit and intermediate-pressure study of the
n2D fluorescence relaxation rate.
We describe the generation and properties of a cryogenic beam of
7Li
2 dimers from sublimation of a neon matrix where lithium atoms have been implanted via laser ablation of solid precursors of metallic lithium or lithium hydride (LiH). Different sublimation regimes lead to pulsed molecular beams with different temperatures, densities and forward velocities. With laser absorption spectroscopy these parameters were measured using the molecular
7Li
2 (R) transitions A$^1Σ_u^+(v'=4,J'=J''+1)\leftarrow
X^1Σ_g^+ (v''=0,J''=0,1,3)
.Inatypicalregime,sublimatingamatrixat16K,translationaltemperaturesof6−−8Kwithadriftvelocityof130m\,
s^{-1}$ in a free expanding pulsed beam with molecular density of 10
9 cm
−3, averaged along the laser axis, were observed. Rotational temperatures around 5--7 K were obtained. In recent experiments we were able to monitor the atomic Li signal -- in the D2 line -- concomitantly with the molecular signal in order to compare them as a function of the number of ablation pulses. Based on the data and a simple model, we discuss the possibility that a fraction of these molecules are being formed in the matrix, by mating atoms from different ablation pulses, which would open up the way to formation of other more interesting and difficult molecules to be studied at low temperatures. Such a source of cryogenic molecules have possible applications encompassing fundamental physics tests, quantum information studies, cold collisions, chemistry, and trapping.
We report the direct observation of
6Li+,
7Li+,
13Li2+,
14Li2+ arising from single collisions between beams of lithium atoms in the presence of a laser field. Although the relative intensity of atomic ions reflects the natural isotopic abundance, a pronounced enrichment of
13Li2+ relative to
14Li2+ is observed.
Collisional transfers induced in the n=6--13 Li Rydberg states by
H2,
N2, and CO molecules have been experimentally studied in a heat-pipe oven. The l-mixing cross sections have been deduced from the fast component of the population relaxations following pulsed laser excitation. The order of magnitude of these cross sections (
103 A
˚2) is well predicted by the existing theories. For large n, the cross sections stay approximately constant. This can be partly explained by the influence of inelastic processes such as n changing. In the particular case of
H2, n-changing collisions have been identified by the fluorescence starting from the n'=n\ifmmode\pm\else\textpm\fi{}1, n-2 levels. Cross sections of the order of 10 A
˚2 have been measured. The systematic study of the n-changing cross sections as a function of the transferred energy shows some evidence of resonance phenomena corresponding to an energy exchange between the Rydberg electron and the molecular rotation. This effect is also observed when
H2 is replaced by
D2. These resonances are well described by the theory of Petitjean and Gounand based on the e-quadrupolar interaction.