State-Selective Probing of CO2 Autoionizing Inner Valence Rydberg States with Attosecond Extreme Ultraviolet Four-Wave Mixing Spectroscopy

Nonlinear spectroscopies can disentangle spectra that are congested due to inhomogeneous broadening. In conjunction with theoretical calculations, attosecond extreme ultraviolet (XUV) four-wave mixing (FWM) spectroscopy is utilized here to probe the dynamics of autoionizing, inner valence excited Rydberg states of the polyatomic molecule, CO2. This tabletop nonlinear technique employs a short attosecond XUV pulse train and two noncollinear, few-cycle near infrared pulses to generate background-free XUV wave-mixing signals. FWM emission is observed from the n = 5 – 7 states of the Henning sharp ndσ_gRydberg series that converges to the ionic B ̃ (_ ^2)Σ_u^+ state. However, these transient emission signals decay with lifetimes of 33 ± 6 fs, 53 ± 2 fs, and 94 ± 2 fs, respectively, which calculations show are consistent with the lifetimes of the short-lived n = 6 – 8 members of the nsσ_g character Henning diffuse Rydberg series. The oscillator strengths of transitions between states involved in all possible resonant FWM processes are calculated, verifying that the nonlinear spectra are dominated by pathways described by an initial excitation to the diffuse nsσ_g Rydberg series and emission from the sharp ndσ_g Rydberg series. The results substantiate not only that attosecond XUV FWM spectroscopy produces rigorous and meaningful measurements of ultrafast dynamics in polyatomic systems, but also that nonlinear spectroscopic techniques are versatile tools to selectively probe dynamics that are otherwise difficult to access.