Technical Details
We investigate electronic excited state dynamics in gas phase molecules using ultrafast core-to-valence transient absorption spectroscopy. This technique combines element specificity with sensitivity to local bonding environment and spin state. In this experiment, an ultraviolet pump pulse electronically excites the molecule of interest, and an extreme ultraviolet (XUV) or soft X-ray probe pulse monitors the dynamics by effecting core-to-valence transitions.
XUV and soft X-rays are produced via high harmonic generation by focusing <40 femtosecond duration 800 nm and 400 nm pulses (for XUV) or near-infrared pulses from a commercial optical parametric amplifier (for soft X-rays) to high intensities in a semi-infinite gas cell filled with a rear gas (He, Ne, or Ar) held at pressures of tens to hundreds of Torr. The generated XUV pulses span <30 eV to >110 eV and the soft X-rays reach ~300 eV. The XUV/soft X-rays are first sent through metal filters to remove near infrared and visible wavelengths and are then focused by a grazing incidence toroidal mirror into a sample cell that contains ambient vapors of the gas phase molecules under study. The transmitted XUV/soft X-rays are then spectrally analyzed in a home-built X-ray spectrometer equipped with a concave grating and an X-ray CCD camera. The time resolution of this apparatus is ~70 fs when using 266 nm pump pulses for initiating the dynamics.
We have employed XUV pulses to investigate excited state dynamics during primary C-I homolysis in acetyl iodide [H3C-C(O)-I] via the iodine 4d-to-valence transitions. This study reported short-lived 4d-to-5p and 4d-to-6p spectral signatures—arising from progressively weaker interactions of the iodine 5p and 6p orbitals with the orbitals on the acetyl radical—as the C-I bond elongates on the dissociative surface to ultimately form an iodine atom and an acetyl radical. We also employed XUV pulses to understand the excited state dynamics in Fe(CO)5 via Fe 3p-to-valence transitions, where our work revealed first spectroscopic signatures of dissociative metal-centered states and singlet and triplet states involved in the sequential ligand dissociation following 266 nm excitation.
Figure 1: Photograph of the apparatus for high harmonic generation and XUV/soft X-ray transient absorption spectroscopy.
Figure 2: (Left) Short-lived iodine 4d-to-valence transitions during C-I bond dissociation in acetyl iodide following 266 nm excitation. See https://doi.org/10.1021/acs.jpca.3c01414. (Right) Spectroscopic detection of singlet and triplet states of Fe(CO)x during sequential CO ligand loss after 266 nm excitation of Fe(CO)5. See https://doi.org/10.1021/jacs.4c07523.
Collaborators
Martin Head-Gordon (University of California, Berkeley)
Anna Krylov (University of Southern California)
Laura McCaslin (Sandia National Laboratories)
Key Contributions
- Ultrafast core-to-valence transient absorption spectroscopy has shed light on complex excited state dynamics in gas-phase molecules, revealing spectroscopic signatures of transiently populated electronic states.
Representative Publications:
J. Troß, K. Carter-Fenk, N. C. Cole-Filipiak, P. Schrader, M. Word, L. M. McCaslin, M. Head-Gordon, K. Ramasesha; Excited state dynamics during primary C−I homolysis in acetyl iodide revealed by ultrafast core-level spectroscopy, Journal of Physical Chemistry A 127 (18), 4103-4114 (2023) https://doi.org/10.1021/acs.jpca.3c01414
J. Troß, J. E. Arias-Martinez, K. Carter-Fenk, N. C. Cole-Filipiak, P. Schrader, L. M. McCaslin, M. Head-Gordon, K. Ramasesha; Femtosecond core-level spectroscopy reveals involvement of triplet states in the photodissociation of gas-phase Fe(CO)5, Journal of the American Chemical Society, 146 (32), 22711-22723 (2024)https://doi.org/10.1021/jacs.4c07523
PI: Krupa Ramasesha