Nuclear quantum dynamics on the ground electronic state of neutral silver dimer 107Ag109Ag probed by femtosecond NeNePo spectroscopy

Abstract

The nuclear quantum dynamics on the ground electronic state of the neutral silver dimer ¹⁰⁷Ag¹⁰⁹Ag are studied by femtosecond (fs) pump–probe spectroscopy using the ‘negative ion – to neutral – to positive ion’ (NeNePo) excitation scheme. A vibrational wave packet is prepared on the X¹Σ⁺_g state of Ag₂via photodetachment of mass-selected, cryogenically cooled Ag₂⁻ using a first ultrafast pump laser pulse. The temporal evolution of the wave packet is then probed by an ultrafast probe pulse via resonant multiphoton ionization to Ag₂⁺. Frequency analysis of the fs-NeNePo spectra obtained for a single isotopologue and pump–probe delay times up to 60 ps yields the harmonic (ω_e = 192.2 cm⁻¹), quadratic anharmonic (ω_ex_e = 0.637 cm⁻¹) and cubic anharmonic (ω_ey_e = 3 × 10⁻⁴ cm⁻¹) constants for the X¹Σ⁺_g state of neutral Ag₂. The fs-NeNePo spectra obtained at different pump wavelengths provide insight into the excitation mechanism. At a pump wavelength of 510 nm instead of 1010 nm, resonant excitation of a short-lived electronically excited state of the anion followed by autodetachment results in population of higher-energy vibrational levels of the neutral ground state. In contrast, at 1140 nm dynamics with a slightly shorter beating period and different relative phase are observed. The present study demonstrates that isotopologue-specific fs-NeNePo spectroscopy provides accurate vibrational constants of mass-selected neutral clusters in their electronic ground state in the terahertz spectral region, which remains difficult to obtain directly in the frequency domain with any other type of spectroscopy of comparable sensitivity.