Ultrafast Infrared Spectroscopy

Technical Details

We investigate vibrational dynamics in electronic ground and excited states of gas phase and condensed phase systems using ultrafast infrared transient absorption spectroscopy. These experiments may utilize ultraviolet or narrowband infrared pump pulses to initiate the dynamics on an electronic excited or the electronic ground state of a system, respectively. These dynamics are then probed using either broadband infrared (BBIR) or narrowband infrared (NBIR) pulses.

BBIR pulses are generated from a laser-driven plasma produced by tightly focused femtosecond pulses at 800 nm, 400 nm, and 267 nm in flowing N2. The BBIR spectrum, detected on a liquid nitrogen-cooled HgCdTe array, spans <700 cm-1 to ~3500 cm-1 (Figure 1); such a large spectral bandwidth allows simultaneous probing of all infrared-active vibrations in the mid-infrared region of the spectrum, representing a significant advantage over the relatively narrow bandwidth output from conventional femtosecond mid-infrared optical parametric amplifiers. Furthermore, the sub-100 femtosecond duration of the BBIR pulses allows exquisite time resolution for following ultrafast vibrational dynamics. For experiments that require low noise levels, we use NBIR probe pulses (tunable from 2.6 to 8 mm wavelength) generated from a commercial optical parametric amplifier and a home-built difference frequency generation set up. 

We have employed ultrafast BBIR spectroscopy to investigate vibrational energy transfer dynamics in solid energetic materials following narrowband infrared excitation of select high-frequency vibrations in the material. Energetic materials exhibit numerous anharmonically coupled vibrations across the mid-infrared region; BBIR pulses enabled probing these myriad vibrations simultaneously, revealing global energy transfer dynamics occurring on sub-picosecond to hundreds of picosecond timescales. We employed ultraviolet pump-NBIR probe transient absorption spectroscopy in our investigations of ligand dissociation dynamics in gas-phase transition metal carbonyls, where the bound C≡O stretching vibrations were used as reporters of the time-dependent coordination changes of the transition metal centers. Our work revealed spectroscopic evidence for key photodissociation intermediates and unraveled far richer photodissociation pathways than previously thought.

Figure 1
Figure 1
Figure 2
Figure 2

Figure 1 BBIR pulse spectrum detected on a HgCdTe array detector. The dips near 2500 cm-1 and 1650 cm-1 are due to absorption from atmospheric CO2 and H2O, respectively.

Figure 2 Transient absorption spectra of RDX pumped at 6.5 mm wavelength and probed with BBIR. This example data shows one of several spectral windows acquired to cover the full mid-infrared region. See https://doi.org/10.1021/acs.jpca.1c04800.

Collaborators

Mitchell Wood (Sandia National Laboratories)

Robert Knepper (Sandia National Laboratories)

William Bassett (Sandia National Laboratories)

Laura McCaslin (Sandia National Laboratories)

Key Contributions

  • Application of ultrafast broadband infrared spectroscopy has led to breakthrough studies of vibrational energy transfer in thin-film energetic materials.

Representative Publications:

N. C. Cole-Filipiak, R. Knepper, M. Wood, K. Ramasesha; Sub-Picosecond to Sub-Nanosecond Vibrational Energy Transfer Dynamics in Pentaerythritol Tetranitrate, Journal of Physical Chemistry Letters 11, 6664-6669 (2020) https://doi.org/10.1021/acs.jpclett.0c01780

N. C. Cole-Filipiak, R. Knepper, M. Wood, K. Ramasesha; Mode-Selective Vibrational Energy Transfer Dynamics in 1,3,5-Trinitroperhydro-1,3,5-Triazine (RDX) Thin Films, Journal of Physical Chemistry A 125 (36) 7788-7802 (2021) https://doi.org/10.1021/acs.jpca.1c04800

  • Ultrafast infrared spectroscopy following ultraviolet electronic excitation has yielded detailed understanding of the complex photodissociation mechanisms in gas-phase transition metal carbonyls.

Representative publications:

N. C. Cole-Filipiak, J. Troß, P. E. Schrader, L. M. McCaslin, K. Ramasesha; Ultraviolet Photodissociation of Gas-Phase Iron Pentacarbonyl Probed with Ultrafast Infrared Spectroscopy, Journal of Chemical Physics 154 (13), 134308(2021) https://doi.org/10.1063/5.0041074

DOE Office of Science Highlight: “Watching Light Breakdown a Model Photocatalyst in Near Real Time” https://www.energy.gov/science/bes/articles/watching-light-break-down-model-photocatalyst-near-real-time

N. C. Cole-Filipiak, J. Troß, P. Schrader, L. M. McCaslin, K. Ramasesha; Ultrafast infrared transient absorption spectroscopy of gas-phase Ni(CO)4 photodissociation at 261 nm, Journal of Chemical Physics 156 (15), 144306 (2022) https://doi.org/10.1063/5.0080844

N. C. Cole-Filipiak, J. Troß, P. Schrader, L. M. McCaslin, K. Ramasesha; Ultrafast production of NiCO and Ni following 197 nm photodissociation of nickel tetracarbonyl, ACS Physical Chemistry Au, DOI: 10.1021/acsphyschemau.4c00033 (2024), Article ASAP https://doi.org/10.1021/acsphyschemau.4c00033

PI: Krupa Ramasesha