Technical Details

Studies of the infrared and ultraviolet spectroscopy of large molecular ions and complexes are carried out in a versatile, multi-stage mass spectrometer that incorporates a cryogenically cooled octopole ion trap that is cooled to 5 K. Single-conformation IR and UV spectra are recorded using IR-UV double resonance photofragment spectroscopy. The spectrum of the protonated peptide shown below illustrates the IR spectra so obtained, which probe the hydrogen bonded network present in the ion.

Figure 1. Schematic diagram of the cryogenically-cooled ion spectroscopy apparatus.


Figure 2. Single-conformation IR spectrum of the cryo-cooled protonated hexapeptide shown in the inset.

Key Contributions

We are using these methods to study a variety of phenomena, including (i) the way in which ions bind and move inside molecular cavities such as the cycloparaphenylenes, (ii) the electronic excited states of ions of relevance to quantum computing, and (iii) electronic energy transfer in chromophore arrays.

PIs: Timothy Zwier, David Chandler

Technical Details

Broadband chirped-pulse Fourier Transform microwave (CP-FTMW) spectroscopy uses fast digital electronics from the communications industry to create chirped-pulses of microwave radiation that can interrogate rotational transitions spread over wide frequency ranges (2=8 GHz or 8-18 GHz) simultaneously. Using fast digitizers, the free-induction decay from the molecular sample is detected, signal-averaged, and Fast Fourier Transformed to obtain the spectrum in the frequency domain. We combine CP-FTMW detection with a time-of-flight mass spectrometer (TOFMS) that can be used to photo-ionize the same sample mixture under identical conditions, thereby enabling correlations to be drawn between the masses of the reactive intermediates and their microwave transitions (see figure).

Key Contributions

We are using these methods to study free radicals and transient intermediates formed in reactive mixtures of relevance to combustion, atmospheric chemistry, CO2 sequestration, and space. We are also expanding our capabilities to include a cryogenically cooled buffer gas cell designed to make and study reactive intermediates and gas phase complexes with increased sensitivity.

PIs: Timothy Zwier, Nils Hansen, Lenny Sheps

Chirped-pulse Fourier Transform microwave spectrometer

Left: SULI intern Zachary Decker operates the time-resolved broadband cavity-enhanced absorption spectrometer under the mentorship of PI Leonid Sheps. Right: the time-resolved UV absorption spectrum of Criegee intermediate CH2OO.

Sensitive cavity-enhanced absorption spectrometry for gas-phase chemical kinetics

Technical Details

Absorption of photons from the IR to the UV spectral range can selectively probe ro-vibrational and electronic transitions of molecules, allowing for characterization of chemical reaction products and intermediates. Direct absorption spectroscopy is a ubiquitous experimental tool for the study of condensed phases, i.e., liquids, rare gas matrices, and solids. However, in gases the concentrations of reactive molecules are too low to absorb enough light for sensitive detection. Consequently, multi-pass methods are used to provide long effective sample path lengths and increase sensitivity. At the CRF, we have developed a new time-resolved broad-band cavity-enhanced absorption spectrometer for gas-phase chemical kinetics studies.

Our spectrometer is incorporated into a laser photolysis reactor in which a laser pulse initiates reactions at repetition rates 1 – 10 Hz. Continuous probe light from a simple Xe arc lamp builds up between two reflective end mirrors, resulting in path length enhancement of 50x – 100x over the entire near-UV to Visible spectral range. The optical buildup cavity output is analyzed by a custom spectrometer, which maps the wavelength and time dimensions spatially onto a CCD camera. Typically, long-pass absorption experiments are done by scanning either the probe wavelength or the time delay, both of which are slow and labor-intensive. By contrast, our TR-BB-CEAS apparatus records the full spectral time evolution simultaneously for each photolysis laser shot, resulting in rapid multiplexed kinetics measurements with microsecond time resolution.

Key Contributions

  • Measured the UV absorption spectra of several prototypical Criegee intermediates: CH2OO, syn– and anti-CH3CHOO, and CH3C(OO)CHCH2.
  • Used the strong UV absorption bands of Criegee intermediates to probe directly the kinetics of their gas-phase reactions with water, H2O dimer, and key trace atmospheric compounds.

Partners

  • Marsha I. Lester, University of Pennsylvania
  • Carl Percival, Jet Propulsion Laboratory
  • Andrew Orr-Ewing, University of Bristol
  • Dudley Shallcross, University of Bristol
  • Rebecca Caravan, Argonne National Laboratory

PIs: Leonid Sheps, Craig A. Taatjes

High Pressure Fluorescence Assay for Gas Expansion (FAGE) for quantification of short-lived reaction intermediates

Figure 1. High-pressure laminar flow reactor, coupled to FAGE detection chamber.

Technical Details

Many important gas-phase processes, such as combustion or the atmospheric processing of pollutants, are complex networks of chemical reactions that occur simultaneously and involve numerous intermediate and product species. These chemical networks are driven by highly reactive radical intermediates that are difficult to detect and quantify experimentally, especially at elevated pressures. However, the detection of these radicals directly from a reacting gas mixture can provide the key to understanding the entire network.

To probe such radicals and other key chemical species, we are developing a new application for Fluorescence Assay by Gas Expansion (FAGE)—a well-established technique, typically used to detect OH and HO2 radicals in the atmosphere. We sample (expand) a small amount of gas from a high-pressure reactor into a low-pressure chamber, where gas collision frequency is sufficiently low to allow the detection of molecules by laser-induced fluorescence (LIF). We employ either photolysis reactors, in which reactions are initiated by a laser pulse, or fast-flow reactors in which chemistry is initiated thermally, as shown in the schematic (Figure 2). Our apparatus enables versatile detection strategies: some species (e.g., OH and CH2O) are detected directly by LIF; others (e.g., HO2) are converted to OH by reaction with NO after expansion and then detected; still others (e.g., H2O2) are photolyzed after expansion and OH fragment detected.

Key Contributions

  • When completed, this apparatus will quantify reaction intermediates that are not reliably detected by other means, enhancing our ability to comprehensively probe complex reaction networks.
  • Accurate, quantitative measurements of key radicals will provide valuable benchmarks for comparison with detailed chemical models.

PI: Leonid Sheps

Technical Details

We are interested in following ultrafast vibrational and structural dynamics in electronic ground and excited states of gas phase and condensed phase systems. To this end, we have built a laser-driven plasma-based source of broadband infrared (BBIR) pulses for use as probes in transient infrared absorption spectroscopy following infrared or ultraviolet excitation. The BBIR pulse spectrum detected on a liquid nitrogen-cooled HgCdTe array detector spans from ~1000 cm-1 to ~3500 cm-1. The large spectral bandwidth allows simultaneous probing of all infrared-active vibrations in the mid-infrared region of the spectrum, presenting an advantage over the 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.

 

Spectrum of BBIR pulse 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.


Spectrally-resolved cross-correlation between 265 nm and BBIR pulses in a Ge wafer, yielding a cross-correlation full-width-at-half-maximum of ~120 fs.

Key Contribution

  • Application of this technique has allowed breakthrough studies of vibrational dynamics in energetic materials.

PI: Krupa Ramasesha

The Combustion Research Facility has a long history of leadership in laser diagnostics, developing and characterizing optical laser techniques to detect and quantify important gas-phase species, temperature, and particulate matter in temporally and spatially resolved measurements. Research in spectroscopic methods underlies this leadership. CRF researchers are developing capabilities that interrogate molecular species using radiation from the microwave to the X-ray region, probing environments that stretch from isolated molecular ions to species sampled from high-pressure high-temperature reactors.