Detection of Explosive Signature Molecules Using Rotational Raman Spectroscopy
Abstract
In this research we will explore the physical processes associated with the formation of optimally prepared rotational wavepackets in gas phase polyatomic molecules in air using picosecond-duration laser pulses. The ultimate goal is to understand the fundamental phenomena necessary to induce a detectable rotational coherence in air that may be used to increase the limits of gas phase detection by many orders of magnitude. We seek to create a cylindrical volume of coherent rotational wavepacket motion that is up to 20 meters in length and 1 cm in diameter, potentially providing an extraordinarily sensitive gas phase detection method given the N2 scaling (where N is the number of molecules) of signal for coherent phenomena. Such a detection volume would, in principle, provide rapid, wide-area scanning for IEDs. The excitation volume will contain an anticipated >7 orders of magnitude more signature molecules than a typical laser focal volume of ~100 micron in diameter and ~1 cm in length. If the detection concept is correct, we anticipate that part-per-billion to part-per-trillion detection limits, thus enabling a detection capability for low vapor pressure signatures. The key physical mechanism underlying the creation of rotational coherence for these investigations is impulsive Raman scattering, which produces periodic revivals of the molecular alignment, leading to a time-dependent change in the molecular polarizability. Impulsive rotational Raman scattering occurs when the pulse duration of the exciting laser is less than the rotational period of the molecule in question. The rotational periods of the 10-20 atom asymmetric top molecules that are of interest as signature molecules are on the order of 50 to 200 picoseconds. The short-duration laser pulse induces a torque on the molecules in the illumination region that leads to the alignment of a fraction of the molecules in the ensemble and the resulting motion evolves with a well-defined molecular phase after the laser pulse is gone. The alignment causes a macroscopic change in the polarizability of the medium and the magnitude of the induced birefringence depends on the difference in polarizabilities among the principal axes of the molecule. Linear molecules typically have the largest degree of birefringence and exhibit more pronounced patterns of rotational revivals, whereas asymmetric top molecules are more complex with regard to excitation and the subsequent evolution of the rotational wavepacket that is created. After the exciting laser pulse has traversed a medium containing linear molecules, for example, O2 and N2, the molecules continue to realign at times corresponding to quarter fractions of the rotational period T = 1/2Bc (where B is the rotational constant and c is the speed of light) until the wavepacket dephases due to collisions. For non-linear molecules, decoherence can also occur through internal energy redistribution between the other moments of inertia of the molecule. For diatomic molecules, the periodic change in the polarizability of the medium resulting from impulsive rotational Raman excitation imprints massive Stokes and anti-Stokes sidebands on a picosecond probe pulse traversing the medium. The rotational revivals of large molecules, like signatures for IEDs, have never been measured in the gas phase using sensitive transient birefringent techniques.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Aug 12, 2016
- Source ID
- N000141512574
Entities
People
- Robert J Levis
Organizations
- Office of Naval Research
- Temple University
- United States Navy