Robynne Lock

Time-Resolved Dynamics in N2O4 Probed Using High Harmonic Generation

The attosecond time-scale electron-recollision process that underlies high harmonic generation has uncovered extremely rapid electronic dynamics in atoms and diatomics. We showed that high harmonic generation can reveal coupled electronic and nuclear dynamics in polyatomic molecules. By exciting large amplitude vibrations in dinitrogen tetraoxide, we showed that tunnel ionization accesses the ground state of the ion at the outer turning point of the vibration but populates the first excited state at the inner turning point. This state-switching mechanism is manifested as bursts of high harmonic light that is emitted mostly at the outer turning point. Theoretical calculations attribute the large modulation to suppressed emission from the first excited state of the ion. More broadly, these results show that high harmonic generation and strong-field ionization in polyatomic molecules undergoing bonding or configurational changes involve the participation of multiple molecular orbitals.

Molecular recollision interferometry in high harmonic generation

Using extreme-ultraviolet interferometry, we measure pi phase shifts in high harmonics generated from transiently aligned molecules. This data directly reflects the quantum interferences in the electron wavepacket due to the two-center molecular structure.

Direct Measurement of the Angular Dependence of the Single-Photon Ionization of Aligned N2 and CO2 †

By combining a state-of-the-art high-harmonic ultrafast soft X-ray source with field-free dynamic alignment, we map the angular dependence of molecular photoionization yields for the first time for a nondissociative molecule. The observed modulation in ion yield as a function of molecular alignment is attributed to the molecular frame transition dipole moment of single-photon ionization to the X, A and B states of N2(+) and CO2(+). Our data show that the transition dipoles for single-photon ionization of N2 and CO2 at 43 eV have larger perpendicular components than parallel ones. A direct comparison with published theoretical partial wave ionization cross-sections confirms these experimental observations, which are the first results to allow such comparison with theory for bound cation states. The results provide the first step toward a novel method for measuring molecular frame transition dipole matrix elements.

Physics career intentions: The effect of physics identity, math identity, and gender

Although nearly half of high school physics students are female, only 21% of physics bachelor’s degrees are earned by women. Using data from a national survey of college students in introductory English courses (on science-related experiences, particularly in high school), we examine the influence of students’ physics and math identities on their choice to pursue a physics career. Males have higher math and physics identities than females in all three dimensions of our identity framework. These dimensions include: performance/competence (perceptions of ability to perform/understand), recognition (perception of recognition by others), and interest (desire to learn more). A regression model predicting students’ intentions to pursue physics careers shows, as expected, that males are significantly more likely to choose physics than females. Surprisingly, however, when physics and math identity are included in the model, females are shown to be equally likely to choose physics careers as compared to males.

Angular-dependence of molecular photoionization cross-sections studied by time-resolved EUV spectroscopy

We obtain angular data on molecular EUV photoionization. We impulsively align N2 and CO2 and then ionize using high-harmonic pulses. By measuring the ion yield versus delay we extract angle-dependent cross-sections.

Exposure to Underrepresentation Discussion: The Impacts on Women’s Attitudes and Identities

In earlier work, we found that women who reported experiencing the “discussion of underrepresentation” in their high school physics classes were more likely to report a career interest in the physical sciences in college. To explore this effect in more detail, we conducted two double-blind, random-assignment experiments on students enrolled in introductory, algebra-based college physics. In the first, students were randomly assigned to read one of two short essays (one focused on the underrepresentation of women in physics, the other reporting on AMO physics research led by a woman) followed by a uniform set of reflection questions. In the second, another cohort of students was primed on their beliefs about gender differences in physics before undergoing the same treatment (one of two essays and reflection questions). In this paper, we compare the impacts on students’ general science identity, physics identity, and performance gains on selected FMCE problems.

Probing Dynamics in Polyatomic Molecules Using High Harmonic Generation: the Role of Ionization Continua

We show that to understand harmonic generation from molecules undergoing a large change in configuration, multiple ionization continua must be considered. After exciting large amplitude vibrations in an N2O4 dimer, bright bursts of harmonics are emitted at the outer turning point of the vibration.

Direct Measurement of Angle-Dependent Single Photon Ionization of N2 and CO2

We present a novel method for determining the angular dependence of molecular photoionization, by measuring time-dependent ionization yields from transiently aligned molecules. This method allows us to map the angular dependence of nondissociative singlephoton ionization for molecules (N2 and CO2) for the first time.

Observation of Elliptically Polarized High Harmonic Emission from Molecules Driven by Linearly Polarized Light

We perform an accurate polarimetry measurement of high harmonic emission from aligned molecules. Surprisingly, we find that harmonic emission from N2 molecules can be strongly elliptically polarized even when driven by linearly polarized laser fields.

Large amplitude modulation of high-order harmonic generation from vibrationally excited molecules

We observe large vibrationally-induced modulations in high order harmonic conversion in N2O4. We explain this unexpected result as due to the changing electronic structure induced by the vibration, leading to preferential emission at the outer turning point of the vibration.