Electronic Relaxation after Resonant Laser Excitation of Cr in Superfluid Helium Nanodroplets

Abstract

Chromium (Cr) atoms embedded into helium nanodroplets (HeN) are ejected from the droplets upon photoexcitation. During ejection they undergo electronic relaxation resulting in bare Cr atoms in various excited states. In a study of the relaxation process we present absorption spectra observed via laser induced fluorescence and beam depletion as well as dispersed fluorescence spectra and time-resolved fluorescence measurements. Broad and shifted absorption structures were found for the strong z7P° ← a7S3 and y7P° ← a7S3 excitations from the ground state. Emission lines are, in contrast, very narrow, which indicates that fluorescence is obtained from bare excited Cr atoms after ejection. Upon excitation into the y7P2,3,4° states we observed fluorescence from y7P2°, z5P1,2,3°, and z7P2,3,4°, indicating that these states are populated by electronic relaxation during the ejection processes. Relative population ratios are obtained from the intensities of individual spectral lines. Excitation into the z7P2,3,4° states resulted in fluorescence only from z7P2°. Estimates of the time duration of the ejection process are obtained from time-resolved measurements.

Photoinduced molecular dissociation and photoinduced recombination mediated by superfluid helium nanodroplets

“…Since the droplet acts as a heat bath, kinetic energy can be dissipated to prevent ejection. As observed for single Cr atoms in earlier experiments [21][22][23] and as predicted by theoretical studies, 44,45 the ground state (a 7 S 3 ) atom remains solvated inside the droplet. Fig.…”

“…13,14 The location of a dopant is dictated by the interaction with helium (the pair potential) and can be estimated by the dimensionless Ancilotto parameter. This has been observed, for example, for Ag, [16][17][18] Cu, 19,20 Cr, [21][22][23] CF 3 ,24,25 and NO. This has been observed, for example, for Ag, [16][17][18] Cu, 19,20 Cr, [21][22][23] CF 3 ,24,25 and NO.…”

“…[21][22][23] As indicated in Fig. It was shown for single Cr atoms solvated inside He N , that the y 7 P 2;3;4 a 7 S 3 ground state transition is shifted and broadened by about 400 to 600 cm À1 due to the Pauli repulsion of the surrounding helium.…”

“…It was shown for single Cr atoms solvated inside He N , that the y 7 P 2;3;4 a 7 S 3 ground state transition is shifted and broadened by about 400 to 600 cm À1 due to the Pauli repulsion of the surrounding helium. [21][22][23] This mechanism enables a highly efficient ionization of bare atoms with one further laser photon, also through Cr autoionizing states that lie close to the ionization potential (g 5 D, e 3 D). 1, after excitation to y 7 P 2;3;4 the atoms are ejected from the droplet while relaxing to energetically lower states (e.g., z 5 P 1;2;3 ).…”

Laser ionization and spectroscopy of Cu in superfluid helium nanodroplets

“…Most experimental and theoretical studies of metal doped He N are concerning alkali atoms. It is well established that alkali atoms reside on the droplet surface [1,9,15,16] because of their large van der Waals radius, while most other atoms are dissolved inside the droplet [5,6,17–19] . Helium can be viewed as a probe to explore the electronic structure of an excited atom [20] and the solvation of atoms inside He N can be extracted from absorption broadenings and line shifts [1,21] .…”