Spectroscopy, the Next Generation: A Prototype Remote Temporal 3D-IR Raman Spectrometer for Planetary Science

Science Motivation:

Raman spectroscopy is a highly informative, non-destructive, remote technique. However, the interpretation of the underlying chemical features can be ambiguous due to overlapping bands and non-unique spectral features shared by similar compounds. In addition, instruments are often plagued by a lack of sensitivity and poor resolution (prohibiting determination of isotopic abundances). Here, we propose to develop a 3D-IR Raman instrument that could help unambiguously characterize and identify molecular species from icy surfaces. Briefly, a femtosecond (fs) or picosecond (ps) tunable near-IR pump laser is used to selectively populate vibrational levels (e.g., fundamental, overtone or combination bands), and subsequently trace the evolution of intramolecular and intermolecular vibrational energy transfer events, monitored with ps-resolution by a tunable probe laser pulse (anti-stokes Raman spectra). Since the IR pump pulse selectively excites only particular quanta from molecular species, and the Raman probe instantaneously monitors only the spectra that result from the decay of those excited states (first internally, then external to the molecule), this technique should allow for unambiguous identification of molecular species. The pump can be swept across a wide range to demonstrate different excited combination bands which devolve into fundamental bands innate to the chemical species under scrutiny. Over time, neighboring molecules become detectable also. Higher order overtone and combination bands become sufficiently separated in comparison to the bandwidth of the pumping laser-line that the preferential selection of isotopic species (for C, N, and O) should be resolvable. As a gated system, the fluorescence signal can be isolated, or collected. Since the technique is Raman-based, surface enhancement (SERS) can also be employed, expected to increase detection limits down to the parts per trillion level. Here, we emphasize how such an instrument could be beneficial as part of a surveying instrument to analyze a comet prior to a sample return mission where it could provide isotopic information on volatiles and hypervolatile species otherwise lost during such a mission.

Objectives & Methodology:

In year 1, we will construct a fs-3D-IR Raman set-up that leveraging the facilities of Co-I Chini whereby his tunable mid-IR laser set-ups will be combined with a gated Raman system integrated to an ultra-high vacuum set-up (PI Bennett) that can simulate cometary ices.

If successful, in year 2, we will build a ps-3D-IR Raman system with low bandwidth (<20 cm-1) enabling us to investigate the potential of 3D-IR Raman to analyze isotopic signatures of volatiles and hypervolatile species. We will determine the limits of detection for these species.

In year 3, we will determine whether the technique can be utilized on non-cooled samples, such as minerals and meteorites as well as whether surface enhanced Raman spectroscopy (SERS) can be utilized alongside it to increase the limits of detection. Lastly, we will determine the optimal wavelengths of interest and requirements for a breadboard instrument, whereby smaller tunable lasers (e.g., quantum cascade lasers) could even be potentially added to existing gated Raman instruments to leverage some of the benefits of 3D-IR Raman spectroscopy.

Relevance & Significance:

The proposed work is relevant to the PICASSO program (goal is to improve the TRL of low-TRL instruments). The capabilities potentially offered by 3D-IR Raman would clearly be beneficial for many future NASA planetary science missions; however, it has not currently been applied to study samples of planetary interest (TRL 1), nor have the claims made here been validated within a laboratory context to demonstrate proof-of-concept (TRL 3).. The purpose of this study is to assemble a 3D-IR Raman system and demonstrate these capabilities on realistic samples of interest to future missions.

Developing Instrument technology to improve measurements for future planetary science missions

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