Franco Vigliotti

Femtosecond transition state spectroscopy of solids: electronic ‘bubble’ formation in solid hydrogen

We report the femtosecond nuclear dynamics of an electronic ‘bubble’ in solid H2. The bubble formation results from the impulsive excitation of the lowest Rydberg state A2Σ+ (3sσ) of the NO impurity by a UV femtosecond pulse. The evolution of the bubble was followed in real-time by means of a probe pulse which maps the transient configurations by inducing transitions to higher-lying Rydberg states. It is found that the bubble is formed coherently by the collective displacement of the matrix species, without recurrences (coherent or incoherent) of the cage motion. The bubble reaches its final configuration in ∼ 1.5 ps.

Lifetime lengthening of molecular Rydberg states in the condensed phase

We report on fluorescence lifetime lengthenings of molecular Rydberg states in condensed media in the case of the NO molecule trapped in inert gas matrices. In rare gas matrices, the fluorescence of the A 2Σ+ state originates from two types of sites, hereafter called red and main. The red site is considered to be a loose site with more than one vacancy in Ar, Kr, and Xe and an h.c.p. site in Ne matrices. It exhibits a lifetime lengthening with respect to the gas phase of 25% in Kr matrices and 100% in Xe matrices. The main site fluorescence stems from monosubstitutional sites. It exhibits lifetime enhancements of up to 100% when going from Ne to Xe matrices. When, however, the fluorescence quantum yields are taken into account, the lifetime increases from the gas phase value to up to two orders of magnitude in the sequence H2–Ne(D2)–Ar–Kr–Xe. Furthermore, this change in transition moment is not observed in the absorption spectrum. These results stress the influence of the solvent and its microscopic struc...

Structural dynamics in quantum solids. II. Real-time probing of the electronic bubble formation in solid hydrogens

The ultrafast dynamics of electronic bubble formation upon excitation of the A(3sσ) Rydberg state of NO trapped in solid H2 and D2 has been studied by femtosecond pump–probe spectroscopy. The evolution of the spherical bubble is followed in real time by means of a probe pulse, which maps the transient configurations via transitions to higher-lying Rydberg states. It is found that bubble formation is a one-way process and no oscillations of the bubble are observed. In addition, thermalization of the system occurs on the time scale of bubble formation. In the process, there is a net energy flow away from the excited center and 0.55–0.6 eV leave the first shell around the impurity. We directly extract from the experimental data the time dependence of the bubble radius, which we represent by a rising exponential with time constants of 300±50 fs in solid H2 and 410±30 fs in solid D2 to reach a final radius of ∼5 A. This is confirmed by simulations of the transients. The different energy dissipation mechanisms ...

Lineshape analysis of impurity Rydberg transitions in van der Waals solids: derivation of intermolecular potentials

Abstract Fluorescence depletion spectra of the A 2 Σ + (3 sσ ) Rydberg fluorescence of NO in rare-gas and hydrogen matrices are reported in the 0.9–1.3 eV region. Bands due to the Rydberg–Rydberg A 2 Σ + (3 sσ ) –C 2 Π (3 pπ ) and A 2 Σ + (3 sσ ) –D 2 Σ + (3 pσ ) transitions are observed. These spectral features are combined with ground-state absorption data of the A, C and D states and with A-state fluorescence data to generate a set of intermolecular potentials describing the impurity–matrix interaction in each state. The effective mode and harmonic approximations are made and the semiclassical reflection method is used.

Rydberg states in quantum crystals NO in solid H2

Fluorescence, excitation and fluorescence depletion spectra of the lowest Rydberg states of NO trapped in H2 matrices are reported. The absorption bands are shifted by about 0.58 eV to the blue of the gas phase energy. They are strongly broadened and exhibit an asymmetry by a blue wing. The fluorescence bands are significantly narrower, with a red wing, and lie very close to the gas phase energy. The absorption lineshape can be accounted for by the large extension of the ground state wavefunction, due to the strong contribution of the zero point motion in the H2 lattice. The absorption–emission Stokes shift is interpreted in terms of ‘bubble’ formation around the Rydberg excited molecule. A moment analysis of the absorption and emission bands in the harmonic approximation shows that most of the absorption–emission Stokes shift is used up as energy to create the ‘bubble’ around the excited molecule. The fluorescence depletion spectrum yields Rydberg–Rydberg transitions very close to the gas phase energy. This, together with the fluorescence spectra, indicates that the molecule is in a quasi-free, gas-phase-like state in the expanded cage. The excitation spectra and the fluorescence depletion spectra indicate a severe compression of the Rydberg series of NO in H2 matrices, which can be accounted for by a large negative electron affinity of solid H2. Concerning the intramolecular energy relaxation in NO, it is found that the Rydberg↔valence relaxation processes follow much the same pattern as observed in rare gas matrices for the lower valence states. For the higher valence states, a photochemical route is suggested. For the vibrational relaxation by Δv=2 in the A state and for the C–A electronic relaxation, intermolecular energy transfer processes between NO molecules are invoked, which occur in the sub-ns timescale.

Structural dynamics in quantum solids. I. Steady-state spectroscopy of the electronic bubble in solid hydrogens

The structural changes due to formation of electronic bubbles in solid D2 are studied by fluorescence, fluorescence-excitation, and fluorescence-depletion spectroscopy of the lowest Rydberg state, A 2S+(3ss), of the NO impurity. The A X band is strongly blue-shifted (.apprx.0.7 eV) with respect to the gas phase and shows a very broad (full width at half max. .apprx.2000 cm-1) and asym. profile. The shift results from the strong repulsion due to the overlap of the extended Rydberg orbital with the matrix species, while the width and asymmetry are governed by quantum effects on the ground-state intermol. wave function. Fluorescence occurs with large absorption-emission Stokes shifts, bringing the A-state emission energy to its gas-phase value, which indicates a very loose cavity around the excited mol. A line-shape anal. of the A-X absorption and emission bands allows one to ext. 1-dimensional intermol. NO-matrix potentials of both involved states. The authors est. the bubble radius to .apprx.5 .ANG., in good agreement with values from the literature for the bubble radius of the solvated electron. Fluorescence-depletion spectra of the A state are also presented along with the ground-state transitions to the higher C 2P(3pp) and D 2S+(3ps) states. They are used to generate intermol. potentials for the C and D states, which are essential ingredients for ultrafast pump-probe expts. of the bubble dynamics. The results obtained for D2 matrixes are compared with those previously published for H2 matrixes. [on SciFinder (R)]

Rydberg series in condensed matter: a fluorescence depletion experiment

Rydberg-Rydberg transitions are induced by absorption from the lowest Rydberg state of NO A 2S+ (3ss) in Ar matrixes, which is excited by an ArF laser at 193.3 nm. The depopulation of the A(v = 0) level to higher Rydberg states is probed by depletion of its fluorescence. Absorption bands are obsd. in the 1.0-2.0 eV region which are attributed to transitions to the 3p, 3d, 4s and 4p members of the Rydberg series. At higher energies, a continuous absorption structure appears. It is attributed to transitions to the n>4 members of the Rydberg series which are compressed in a tiny energy range below the ionization potential. A quantum defect model is applied and accounts well for the observations. [on SciFinder (R)]

Dynamics of electronic “bubble” formation in solid hydrogen

Excitation of the A(3ss) Rydberg state of NO trapped in an H2 crystal leads to the electronic bubble formation around the NO impurity, resulting in an increase of 0.9 .ANG. of the ground state cage radius. Its dynamics was followed in real-time by femtosecond pump-probe spectroscopy. Bubble formation is completed in .apprx.1.5 ps without recurrences of the cage motion. A model based on classical hydrodynamics provides quant. agreement with exptl. data, suggesting that solid H can be described like a liq. on ultrashort time scales. [on SciFinder (R)]