US EPA

2117872 

Journal Article 

Photoionization of isooctane and n-octane in intense laser fields: The effect of irradiance on ionization rates and electron dynamics 

Healy, AT 

2008 

0 

Dissertation Abstracts International, B: The Sciences and Engineering
ISSN: 0419-4217 

University Microfilms International, P.O. Box 1764, Ann Arbor, MI, 48106, USA 

69 

1 (2008) 

Thin path length jets (60 mum) of liquid isooctane and n-octane have been photoionized with 36-70 fs pulses of 3.1 eV photons. The population of electrons ejected post ionization is investigated over a large range of ionizing irradiance, I(ex), though transient absorption (TA) measurements at wavelengths in the range 570 nm (2.17 eV) to 1315 nm (0.94 eV). As I(ex) is varied over a range from 3 TW/cm2 to 410 TW/cm2, the dependence of the TA intensity on I(ex) at time delays 0.7 ps and 2.5 ps exhibits the periodic structure theoretically predicted to develop as a result of multiphoton channel closings. At low I(ex) ( 9 TW/cm2, n declines with increasing I(ex) up to I(ex) = 13 TW/cm2, at which point n abruptly increases to 4. The pattern is repeated at I(ex) > 13 TW/cm 2, albeit with n declining from 4 and then abruptly increasing to 5 as I(ex) becomes greater than 100 TW/cm2. A similar trend is observed in n-octane. The decay of the TA intensity in both liquids has been measured from 0.50 ps to 180 ps over the same range of irradiance. Via comparison of the two liquids, and electron quenching studies, the TA at wavelengths longer than 800 nm has been assigned to be predominantly due to absorption by the electron. At the lowest irradiances, where n = 3 photons are required for photoionization, the TA decay in isooctane is characteristic of a geminate ion pair decaying via diffusive recombination in a Coulomb field. As the irradiance is increased, an early time, rapid, exponential decay of the TA begins to develop until an irradiance is reached (~ 13 TW/cm2) at which our studies indicate that the n = 3 channel closes. At this irradiance, the TA decay returns to purely diffusive-like. As the irradiance is further increased, there is a reappearance of the early time exponential decay until the n = 4 channel closes (~ 100 TW/cm2) when again the exponential disappears and the temporal behavior returns to diffusive-like. With further increase of the irradiance, the exponential decay reappears. The dependence of the TA intensity on I(ex), and possible origins for the periodic behavior in the TA decay, are compared to the non-perturbative, strong field approximation (SFA) for ionization in intense radiation fields developed by Reiss [Phys. Rev A 22, 1786 (1980)]. Electron-ion recombination has been simulated under a variety of conditions via the Monte Carlo method to explore the influence of ion pair densities and electron thermalization distributions on electron survival probabilities in liquid isooctane. 

Irradiance; Photoionization; Thresholds; Time delay; Decay; Diffusion; Density; Trends; Isooctane; Ionization; Tantalum; Computer simulation; Liquids; Wavelengths; Ion pairs; Quenching; Photons; Channels; Coulomb friction