Resonant UV-fs-TCFWM spectroscopy on formaldehyde.

The first electronically excited singlet A 1A2 state of formaldehyde (H2CO) is the origin of different competing dissociation channels. An all-ultraviolet (UV) electronically resonant variant of time-resolved fs-two-colour four-wave mixing (fs-TCFWM) spectroscopy has been applied to monitor the dynamics of rovibrational manifolds in this state. The method provides a variety of options to probe molecular dynamics according to the temporal sequence of dipole transitions involved. Signal contributions arising from different molecular states can partially be distinguished by specific sequences of the applied laser pulses. A dispersed signal detection method reveals additional information that is often inaccessible using spectrally integrated detection. We discuss UV-fs-TCFWM experiments which involve resonant transitions to the rovibrational manifolds of the C-O stretch and out-of-plane combination bands in the first electronically excited singlet state of H2CO at energies close to the HCO production threshold. The experimental results are compared to simulations.