Coherent Phonons in Antimony: an Undergraduate Physical Chemistry Solid-State Ultrafast Laser Spectroscopy Experiment

Ultrafast laser pump-probe spectroscopy is an important and growing field of physical chemistry that allows the measurement of chemical dynamics on their natural timescales, but undergraduate laboratory courses lack examples of such spectroscopy and the interpretation of the dynamics that occur. Here we develop and implement an ultrafast pump-probe spectroscopy experiment for the undergraduate physical chemistry laboratory course at the University of California Berkeley. The goal of the experiment is to expose students to concepts in solid-state chemistry and ultrafast spectroscopy via classic coherent phonon dynamics principles developed by researchers over multiple decades. The experiment utilizes a modern high-repetition rate 800 nm femtosecond Ti:Sapphire laser, split pulses with a variable time delay, and sensitive detection of transient reflectivity signals. The experiment involves minimal intervention from students and is therefore easy and safe to implement in the laboratory. Students first perform an intensity autocorrelation measurement on the femtosecond laser pulses to obtain their temporal duration. Then, students measure the pump-probe reflectivity of a single-crystal antimony sample to determine the period of coherent phonon oscillations initiated by an ultrafast pulse excitation, which is analyzed by fitting to a sine wave. Due to the disruption of in-person instruction caused by the COVID-19 pandemic, during those semesters students were provided the data they would have obtained during the experiment to analyze at home. Evaluation of student written reports reveals that the learning goals were met, and that students gained an appreciation for the field of ultrafast laser-induced chemistry

Coherent Phonons in Antimony: an Undergraduate Physical Chemistry Solid-State Ultrafast Laser Spectroscopy Experiment

Ultrafast laser pump-probe spectroscopy is an important and growing field of physical chemistry that allows the measurement of chemical dynamics on their natural timescales, but undergraduate laboratory courses lack examples of such spectroscopy and the interpretation of the dynamics that occur. Here we develop and implement an ultrafast pump probe spectroscopy experiment for the undergraduate physical chemistry laboratory course at the University of California Berkeley. The goal of the experiment is to expose students to concepts in solid-state chemistry and ultrafast spectroscopy via classic coherent phonon dynamics principles developed by researchers over multiple decades. The experiment utilizes a modern high-repetition-rate 800 nm femtosecond Ti:Sapphire laser, split pulses with a variable time delay, and sensitive detection of transient reflectivity signals using the lock-in technique. The experiment involves minimal intervention from students and is therefore easy and safe to implement in the laboratory. Students first perform an intensity autocorrelation measurement on the femtosecond laser pulses to obtain their temporal duration. Then, students measure the pump-probe reflectivity of a single-crystal antimony sample to determine the period of coherent phonon oscillations initiated by an ultrafast pulse excitation, which is analyzed by fitting to a sine wave. Students who completed the experiment in-person obtained good experimental results, and students who took the course remotely due to the COVID-19 pandemic were provided with the data they would have obtained during the experiment to analyze. Evaluation of student written and oral reports reveals that the learning goals were met, and that students gained an appreciation for the field of ultrafast laser-induced chemistry

Attosecond kinetics of photoexcited germanium

Attosecond transient reflectivity is developed to observe the photoexcitation dynamics in germanium. Attosecond time-resolved measurements of the dielectric function reveal a fewfemtosecond collective electronic response time, which renormalizes the Coulomb interaction between the excited carriers

Attosecond kinetics of photoexcited Germanium

Attosecond transient reflectivity is developed to observe the photoexcitation dynamics in germanium. Attosecond time-resolved measurements of the dielectric function reveal a fewfemtosecond collective electronic response time, which renormalizes the Coulomb interaction between the excited carriers

Solid-state core-exciton dynamics in NaCl observed by tabletop attosecond four-wave mixing spectroscopy

Nonlinear wave-mixing in solids with ultrafast x-rays can provide new insight into complex electronic dynamics of materials. Here, tabletop-based attosecond noncollinear four-wave mixing (FWM) spectroscopy using one extreme ultraviolet (XUV) pulse from high harmonic generation and two separately timed few-cycle near-infrared (NIR) pulses characterizes the dynamics of the Na+ L2,3 edge core-excitons in NaCl around 33.5 eV. An inhomogeneous distribution of core-excitons underlying the well-known doublet absorption of the Na+ \Gamma-point core-exciton spectrum is deconvoluted by the resonance-enhanced nonlinear wave-mixing spectroscopy. In addition, other dark excitonic states that are coupled to the XUV-allowed levels by the NIR pulses are characterized spectrally and temporally. Approximate sub-10 femtosecond coherence lifetimes of the core-exciton states are observed. The core-excitonic properties are discussed in the context of strong electron-hole exchange interactions, electron-electron correlation, and electron-phonon broadening. This investigation successfully indicates that tabletop attosecond FWM spectroscopies represent a viable technique for time-resolved solid-state measurements

Decomposition of contributions from core-levels exhibiting spin-orbit splitting in XUV core-level spectroscopy

Attosecond transient absorption spectroscopy (ATAS) is a versatile technique that allows observing ultrafast charge dynamics in solid-state samples. In ATAS, the transient absorption of a core-level excitation in the extreme UV (XUV) is measured following optical excitation of the sample. The lack of spin-selectivity in the XUV pulse results in overlapping XUV absorption spectra from spin-orbit split core-levels and leads to difficulty in disentangling the spectral signatures. Here, we demonstrate the successful retrieval of the contribution of a single spin-orbit level on the XUV transient absorption signal. Under the approxn. that the spin-orbit split levels yield identical absorption spectra which are energetically shifted and weighted by the degeneracy of the spin-orbit states, the contribution of one single spin-orbit state can be retrieved by the Fourier transform of the measured signal. In this case the energy shift of the spectra from the spin-orbit split states becomes a phase shift. By dividing out this phase factor and taking the inverse Fourier transform, the underlying signal referenced to a single spin-orbit level can be retrieved. We apply this method to an ATAS measurement at the germanium M_(4,5)-edge (30 eV), where the spin-orbit energy splitting (0.57 eV) is comparable to the germanium band gap (0.66 eV). The successful decompn. of the transient absorption signals yields clear, spectrally-resolved signatures of electrons and holes such that carrier dynamics can be simultaneously measured and characterized. The presented method allows decompg. contributions of spin-orbit split core-levels in a transient absorption spectrum. This allows clearer assignment of spectroscopic features and reveals weak signal contributions not visible in the exptl. raw data. In the presented case of germanium M-edge spectroscopy, a clear assignment of features assocd. with electrons, holes and the bandgap becomes possible only after applying this method

Decomposition of contributions from core-levels exhibiting spin-orbit splitting in XUV core-level spectroscopy

Attosecond transient absorption spectroscopy (ATAS) is a versatile technique that allows observing ultrafast charge dynamics in solid-state samples. In ATAS, the transient absorption of a core-level excitation in the extreme UV (XUV) is measured following optical excitation of the sample. The lack of spin-selectivity in the XUV pulse results in overlapping XUV absorption spectra from spin-orbit split core-levels and leads to difficulty in disentangling the spectral signatures. Here, we demonstrate the successful retrieval of the contribution of a single spin-orbit level on the XUV transient absorption signal. Under the approxn. that the spin-orbit split levels yield identical absorption spectra which are energetically shifted and weighted by the degeneracy of the spin-orbit states, the contribution of one single spin-orbit state can be retrieved by the Fourier transform of the measured signal. In this case the energy shift of the spectra from the spin-orbit split states becomes a phase shift. By dividing out this phase factor and taking the inverse Fourier transform, the underlying signal referenced to a single spin-orbit level can be retrieved. We apply this method to an ATAS measurement at the germanium M_(4,5)-edge (30 eV), where the spin-orbit energy splitting (0.57 eV) is comparable to the germanium band gap (0.66 eV). The successful decompn. of the transient absorption signals yields clear, spectrally-resolved signatures of electrons and holes such that carrier dynamics can be simultaneously measured and characterized. The presented method allows decompg. contributions of spin-orbit split core-levels in a transient absorption spectrum. This allows clearer assignment of spectroscopic features and reveals weak signal contributions not visible in the exptl. raw data. In the presented case of germanium M-edge spectroscopy, a clear assignment of features assocd. with electrons, holes and the bandgap becomes possible only after applying this method

Solid-state core-exciton dynamics in NaCl observed by tabletop attosecond four-wave mixing spectroscopy

Nonlinear wave-mixing in solids with ultrafast x-rays can provide new insight into complex electronic dynamics of materials. Here, tabletop-based attosecond noncollinear four-wave mixing (FWM) spectroscopy using one extreme ultraviolet (XUV) pulse from high harmonic generation and two separately timed few-cycle near-infrared (NIR) pulses characterizes the dynamics of the Na+ L2,3 edge core-excitons in NaCl around 33.5 eV. An inhomogeneous distribution of core-excitons underlying the well-known doublet absorption of the Na+ \Gamma-point core-exciton spectrum is deconvoluted by the resonance-enhanced nonlinear wave-mixing spectroscopy. In addition, other dark excitonic states that are coupled to the XUV-allowed levels by the NIR pulses are characterized spectrally and temporally. Approximate sub-10 femtosecond coherence lifetimes of the core-exciton states are observed. The core-excitonic properties are discussed in the context of strong electron-hole exchange interactions, electron-electron correlation, and electron-phonon broadening. This investigation successfully indicates that tabletop attosecond FWM spectroscopies represent a viable technique for time-resolved solid-state measurements