The predissociation of highly excited states in acetylene by time-resolved photoelectron spectroscopy

Abstract

We study the dynamics of highly excited states in acetylene initiated by an ultrashort vacuum ultraviolet laser pulse. Electronic states lying in the 4s-3d Rydberg region are excited with one femtosecond pulse, and the dynamic development of the states is monitored by a second short pulse which ionizes the system. We show that even for femtosecond pulses where the bandwidth of the exciting pulse covers several electronic states, it is possible to extract short decay lifetimes through time-resolved photoelectron spectroscopy by using a frequency-modulated ͑chirped͒ excitation pulse. We report decay lifetimes for the F 4 0 2 and E 4-5 0 2 states in acetylene, and for the E 4 0 2 and E 5 0 2 states in d-acetylene. The time evolution measured in the electron spectra is compared to decay spectra measured using ion yield and the differences in these results are discussed.

Femtosecond Time-Resolved Photoelectron Spectroscopy

“…170 The F 1 Σ u + (3dπ g ) Rydberg state absorbs most strongly in this region and exhibits symmetric stretching excitation of the C-C triple bond. In this experiment, these researchers doubled their high-power Ti:sapphire pulse to 396 nm and then used third harmonic generation of this in xenon to generate fs VUV pulses tunable around 132 nm (9.4 eV).…”

High-order harmonic generation from highly excited states in acetylene

“…It is widely known that resonance phenomena greatly influence photoabsorption and photoionization cross sections in acetylene [20][21][22][23][24][25][26][27][28]. While many studies have considered excitations from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO), there is much interest in the role of highly excited states in photoabsorption and photoionization at vacuum ultraviolet (VUV) wavelengths [23][24][25][26][27][28].…”

Probing the role of excited states in ionization of acetylene

“…However, the cut-off energy for this inner plateau is lower than that predicted by the classical three-step model: we attribute this to suppression of ionization from the LUMO+1 due to transitions back to the HOMO-1 [66]. These transitions from the LUMO+1 to the HOMO-1 give rise to a window of enhanced harmonics (harmonics [11][12][13][14][15][16][17][18][19][20][21]. We can show that the inner plateau is due to the 3σ g → 3σ u transition rather than the 2σ u → 4σ g transition by calculating the contribution of each state to the harmonic response using Eq.…”

“…More recently, several studies have considered the response to strong fields. This has included single [16,17] and double [18,19] ionization, dissociative ionization of highly excited states [20], high harmonic generation [21][22][23][24] and more recently initiating isomerization of acetylene to vinylidene [25].…”