High Harmonic Generation from Rotationally Excited Molecules

High harmonic generation (HHG) is understood through a three-step model. A strong laser field ionizes an atom or molecule. The free electron propagates in the laser field and may recombine with the atom or molecule leading to the generation of extreme ultraviolet or soft x-ray light at odd harmonics of the fundamental. Since the wavelength of the recombining electron is on the order of internuclear distances in molecules, HHG acts as a probe of molecular structure and dynamics. Conversely, control of the molecules leads to control of the properties (intensity, phase, and polarization) of the harmonic emission. Rotationally exciting molecules provides field-free molecular alignment at time intervals corresponding to fractions of the rotational period of the molecule. Alignment is necessary for understanding how the harmonic emission depends on molecular structure and alignment. Additionally, HHG acts as a probe of the rotational wavepackets. This thesis reports three experiments on HHG from rotationally excited molecules. Before we can use HHG as a probe of complex molecular dynamics or control harmonic properties through molecules, the harmonic emission from aligned, linear molecules must first be understood. To that end, the first experiment measures the intensity and phase of harmonics generated from N2O and N2 near times of strong alignment revealing interferences during recombination. The second experiment demonstrates HHG as a sensitive probe of rotational wavepacket dynamics in CO2 and N2O, revealing new revival features not detected by any other probe. The final experiment focuses on understanding and controlling the polarization state of the harmonic emission. Generating elliptically polarized harmonics would be very useful for probing molecular and materials systems. We observe an elliptical dichroism in polarization-resolved measurements of the harmonic emission from aligned N2 and CO2 molecules, revealing evidence for electron-hole dynamics between the times of ionization and recombination

Impact of out-of-class science and engineering activities on physics identity and career intentions

The number of physics bachelor’s degrees that are awarded in the United States annually is small compared to most other science, technology, engineering, and mathematics fields, and only about one-fifth of these degrees are awarded to women. Understanding the influence of students’ science and engineering experiences on career choices is critical in order to improve future efforts to increase the number of physics majors and the participation of women. In this work, we use a physics identity framework to examine the impact of out-of-class science and engineering activities on three identity dimensions and the relationship between these dimensions and physics career choice. Through structural equation modeling of survey data from 5541 college students, we find that out-of-class science and engineering activities have the largest influence on physics performance/competence beliefs, but the association of performance/competence beliefs to overall physics identity and physics career choice is primarily mediated through recognition beliefs and physics interests. Furthermore, out-of-class science and engineering activities have a larger effect on recognition beliefs for men than for women, which is a challenging finding in light of the fact that recognition beliefs are the most influential identity dimension for women. The results of this work begin to highlight the need for out-of-class science and engineering activities that focus on not only enhancing students’ performance/competence beliefs but also students’ interests, particularly those students not previously interested, and women’s recognition beliefs with respect to physics

Identity, critical agency, and engineering majors: An affective model for predicting engineering as a career choice.

Prior to college, many students do not have experience with engineering, but some ultimately choose an engineering career. Additionally, women choose engineering at lower rates than men, which results in women’s underrepresentation. The framework of critical engineering agency (CEA) is utilized to understand student attitudes and beliefs for choosing engineering. We investigate the relationships among students’ math and physics identities in high school that predict choice of engineering careers; how students’ beliefs about science and technology predict a choice of engineering careers; whether these beliefs are different by gender; and how well CEA explains students’ engineering choice. The data were drawn from the nationally representative Sustainability and Gender in Engineering (SaGE) survey distributed during Fall 2011 (n = 6,772). Structural equation modeling (SEM) was used to understand students’ affective beliefs for predicting engineering choice in college. Multiple subject-related identities compose engineering students’ identity at the beginning of college. Recognition from others and interest in a subject are important predictors of developing an identity. Students’ performance/competence alone are not significant predictors of engineering, but are mediated by interest and recognition from others. Student identities and agency beliefs are significant predictors of engineering choice (explaining 20.2% of the variance). Gender differences were found for students’ math and physics identities and agency beliefs. Students’ self-beliefs account for approximately one-fifth of the variance in engineering choice in the transition from high school to college. Steps can be taken to improve students’ affective beliefs in early engineering experiences through addressing identity and agency beliefs

Impact of the Next Gen PET Curriculum on Science Identity

The Next Gen Physical Science and Everyday Thinking (PET) curriculum was designed for physical science courses for future elementary teachers. However, this curriculum may also be used in general education conceptual science courses. The materials are aligned with the Next Generation Science Standards and use a guided-inquiry approach. Next Gen PET is currently being implemented at many universities nationwide. We examine the impact of this curriculum on students’ science identities at a subset of these universities. The identity framework consists of three dimensions. Recognition is the extent to which a student believes that parents, peers, and professors view them as a science person. Interest describes their enjoyment of science. Finally, performance/competence represents a student’s belief in their abilities to understand science and complete science related tasks. The shift in science identities was measured with items adapted from a previously developed physics identity instrument. We found positive shifts in the science identities of students enrolled in face-to-face courses targeted at pre-service teachers using the Next Gen PET curriculum, but a slight negative shift in the science identity of students enrolled in an online course targeted at non-science majors which did not use the Next Gen PET curriculum

Discussing underrepresentation as a means to facilitating female students’ physics identity development

Despite the fact that approximately half of high school physics students are female, only 21% of physics bachelor’s degrees are awarded to women. In a previous study, drawn from a national survey of college students in introductory English courses, five factors commonly proposed to positively impact female students’ choice of a physical science career were tested using multivariate matching methods. The only factor found to have a positive effect was the explicit discussion of the underrepresentation of women in physics. In order to explore this further, a case study of the classes of one teacher reported to discuss the underrepresentation of women was conducted. Two classroom underrepresentation discussions were recorded, students and teacher were interviewed, and relevant student work was collected. Analyzing the case study data using a figured worlds framework, we found that discussing the underrepresentation of women in science explicitly creates an opportunity for students’ figured worlds of professional and school science to change, and facilitates challenging their own implicit assumptions about how the world functions. Subsequently, the norms in students’ figured worlds may change or become less rigid allowing for a new openness to physics identity development amongst female students

Factors that affect the physical science career interest of female students: Testing five common hypotheses

There are many hypotheses regarding factors that may encourage female students to pursue careers in the physical sciences. Using multivariate matching methods on national data drawn from the Persistence Research in Science and Engineering (PRiSE) project (n=7505), we test the following five commonly held beliefs regarding what factors might impact females’ physical science career interest: (i) having a single-sex physics class, (ii) having a female physics teacher, (iii) having female scientist guest speakers in physics class, (iv) discussing the work of female scientists in physics class, and (v) discussing the underrepresentation of women in physics class. The effect of these experiences on physical science career interest is compared for female students who are matched on several factors, including prior science interests, prior mathematics interests, grades in science, grades in mathematics, and years of enrollment in high school physics. No significant effects are found for single-sex classes, female teachers, female scientist guest speakers, and discussing the work of female scientists. However, discussions about women’s underrepresentation have a significant positive effect

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.Peer reviewed: YesNRC publication: Ye

Examining the effect of counternarratives about physics on women’s physics career intentions

Women and many people of color continue to be minoritized in STEM and notably in physics. We conducted two studies demonstrating that exposure to counternarratives about who does physics and why one does physics significantly increases high school students—especially women’s—physics-related career intentions. These counternarratives facilitate making connections with students’ career plans and help in sensemaking causes for the continued minoritization of women in physics. Two separate studies measured the impacts of these interventions on students’ physics-related career intentions: first, with an intentionally selected group of teachers (10 teachers, 823 students) across regions and contexts in the U.S.; second, with a randomly sampled group of teachers (13 teachers, 1509 students) from three regions that also included a comparable control group. The results clearly show the importance of exposure to counternarratives in the development of high school students’ career interests, particularly for women and minoritized racial or ethnic groups, and that such counternarratives may help to address systemic issues of underrepresentation in STEM