Femtosecond extreme ultraviolet ion imaging of ultrafast dynamics in electronically excited helium nanodroplets

Abstract

Abstract-A novel femtosecond extreme ultraviolet (EUV) ion-imaging technique is applied to study ultrafast dynamics in electronically excited helium nanodroplets. Ion mass spectra recorded by single-photon EUV ionization and by transient EUV-pump/IRprobe two-photon ionization differ significantly for EUV photon energies below and above ∼24 eV, in agreement with recently performed synchrotron measurements. Pump-probe time-delaydependent ion kinetic energy (KE) spectra exhibit two major contributions: a decaying high KE component and a rising low KE component, which are attributed to the different excitation regimes. A model is presented that describes the excitation energy dependence of the relaxation and ionization dynamics within the framework of bulk and surface states. The model is supported by recent ab initio calculations on electronically excited states of 25-atom clusters. An intraband relaxation mechanism is proposed that proceeds on a ∼10-20-ps time scale and that corresponds to the transfer of electronic excitation in the Rydberg n = 2 manifold from bulk to surface states.

Extreme ultraviolet ionization of pure He nanodroplets: Mass-correlated photoelectron imaging, Penning ionization, and electron energy-loss spectra

“…[6][7][8][9][10][11] Recently, time-resolved experiments have become possible using femtosecond light pulses in the extreme ultraviolet (EUV) spectral range provided by high-order harmonic generation. [12][13][14][15] Based on the photoionization and dispersed fluorescence emission measurements, the following three distinct regimes of excitation and ionization have been identified: (i) At photon energies 20.5 < hν < 23 eV, He nanodroplets are excited with large cross sections into perturbed excited states ("bands") derived from the 1s2s 1 S and 1s2p 1 P He atomic levels. Fast droplet-induced intraband and inter-band relaxation as well as He * 2 excimer formation follows the excitation.…”

“…Fast droplet-induced intraband and inter-band relaxation as well as He * 2 excimer formation follows the excitation. 9,10,13,14 Due to the repulsive interaction between excited He* or He * 2 and the He environment the excitation migrates to the surface presumably involving both resonant hopping of the a) Current address: IBM-India Semiconductor R&D Center, D3-F1, Manyata Park, Bangalore 560045, India. b) mudrich@physik.uni-freiburg.de electronic excitation as well as nuclear motion of the excited He* atom.…”

“…b) mudrich@physik.uni-freiburg.de electronic excitation as well as nuclear motion of the excited He* atom. 3,[14][15][16] Depending on the size of the He droplet, the He*(1s2p 1 P) state is trapped at the surface and eventually relaxes into the long-lived 1s2s 1,3 S states or into vibrationally excited He * 2 molecules. 2 The latter are subject to vibrational relaxation by coupling to the He droplet and eventually evaporate off the droplet surface.…”

“…In addition to the aforementioned relaxation channels, the emission of He* and He * 2 in Rydberg states dominates, 10,14,15 while the fraction of He * 2 dimers increases with rising excitation energies. 9,10 At hν > 24 eV population of triplet states of He was also observed.…”

“…7,8 Recent time-resolved photoelectron and photoion imaging experiments have revealed the dynamics of various relaxation processes in this regime. [12][13][14][15] (iii) At photon energies hν > 24.6 eV, that is above E i,He , He + ions (positive holes) are created in the droplets. The positive charges subsequently migrate through the He droplet by resonant hopping and eventually localize by forming He + 2 molecular ions or by ionizing a dopant if present.…”