UC Berkeley

Ultrashort pulses with durations less than one femtosecond advance understanding of chemistry and physics by facilitating time-resolved measurements on the timescale of electronic motion. A very pragmatic type of experiment incorporating isolated attosecond (10^&minus18 s) pulses is transient absorption measurements that utilize the broad bandwidth of attosecond pulses. In these measurements the dispersed spectrum of the isolated attosecond pulse is recorded after the attosecond pulse and a time-delayed optical pulse pass through an experimental target of interest. This thesis focuses on the development of an attosecond transient absorption spectrometer with the ultimate goal of using the spectrometer to measure atomic and molecular dynamics. Two experimental studies are presented: one focusing on the behavior of helium atoms bathed in a strong near-infrared optical field, and one focusing on measurement of the lifetimes of autoionizing states in xenon.

Measurement of the absorption spectrum of atomic helium bathed in a near-infrared (780 nm) field exhibits new features when the attosecond pulse and the optical pulse are overlapped in the gas target. The features near the 1s2p transition at 21.21 eV are identified as intermediates in two-photon transitions to reach nearby s and d helium excited states that are inaccessible via one photon transitions from the 1s² ground state of He. Initial measurements identified three intermediate states, and further experiments investigated the behavior of the states as the near infrared field intensity is increased.

Attosecond time-resolved measurement of ultrashort lifetimes of autoionizing states is also investigated. The 5s5p⁶6p and 5s5p⁶7p autoionizing states of xenon are visible in the extreme-ultraviolet absorption spectrum of Xe as Fano resonances. The Fano resonances are suppressed by a time-delayed near-infrared pulse and recover with a time-constant that is related to the lifetime of the state. The studies here identified key parameters for time-resolved measurements of the autoionization lifetime, including spectral resolution of the attosecond transient absorption spectrometer.