Noninvasive detection of concealed explosives: depth profiling through opaque plastics by time-resolved Raman spectroscopy.

The detection of explosives concealed behind opaque, diffusely scattering materials is a challenge that requires noninvasive analytical techniques for identification without having to manipulate the package. In this context, this study focuses on the application of time-resolved Raman spectroscopy (TRRS) with a picosecond pulsed laser and an intensified charge-coupled device (ICCD) detector for the noninvasive identification of explosive materials through several millimeters of opaque polymers or plastic packaging materials. By means of a short (250 ps) gate which can be delayed several hundred picoseconds after the laser pulse, the ICCD detector allows for the temporal discrimination between photons from the surface of a sample and those from deeper layers.

Time-resolved spatially offset Raman spectroscopy for depth analysis of diffusely scattering layers.

The objective of this study is to use time-resolved (TR) Raman spectroscopy, spatially offset Raman spectroscopy (SORS), and a combination of these approaches to obtain high quality Raman spectra from materials hidden underneath an opaque layer. Both TR Raman and SORS are advanced techniques that allow for an increased relative selectivity of photons from deeper layers within a sample. Time-resolved detection reduces fluorescence background, and the selectivity for the second layer is improved.

Picosecond Raman spectroscopy with a fast intensified CCD camera for depth analysis of diffusely scattering media.

A spectroscopic depth profiling approach is demonstrated for layers of non-transparent, diffusely scattering materials. The technique is based on the temporal discrimination between Raman photons emitted from the surface and Raman photons originating from a deeper layer. Excitation was carried out with a frequency-doubled, 3 ps Ti:sapphire laser system (398 nm; 76 MHz repetition rate).

Fluorescence rejection in resonance Raman spectroscopy using a picosecond-gated intensified charge-coupled device camera.

A Raman instrument was assembled and tested that rejects typically 98-99% of background fluorescence. Use is made of short (picosecond) laser pulses and time-gated detection in order to record the Raman signals during the pulse while blocking most of the fluorescence. Our approach uses an ultrafast-gated intensified charge-coupled device (ICCD) camera as a simple and straightforward alternative to ps Kerr gating.

Hexamerization of the bacteriophage T4 capsid protein gp23 and its W13V mutant studied by time-resolved tryptophan fluorescence.

The bacteriophage T4 capsid protein gp23 was studied using time-resolved and steady-state fluorescence of the intrinsic protein fluorophore tryptophan. In-vitro gp23 consists mostly of monomers at low temperature but forms hexamers at room temperature. To extend our knowledge of the structure and hexamerization characteristics of gp23, the temperature-dependent fluorescence properties of a tryptophan mutant (W13V) were compared to those of wild-type gp23.

High resolution slice imaging of a molecular speed distribution.

High resolution slice imaging experiments are reported measuring the speed distribution of molecular fragments, recoiling at a most probable speed v(mp), with a full-width-half-maximum (FWHM) speed resolution near the permille level: FWHM(v)/v(mp) = 1.9 x 10(-3). We implemented a high resolution single-particle slice imaging detector and used a two-colour resonance-enhanced multi-photon ionisation (REMPI) scheme to reduce broadening of the speed distribution due to the electron kick.

Time-resolved fluorescence of the bacteriophage T4 capsid protein gp23.

The time-resolved fluorescence properties of the bacteriophage T4 capsid protein gp23 are investigated. The structural characteristics of this protein are largely unknown and can be probed by recording time-resolved and decay-associated fluorescence spectra and intensity decay curves using a 200 ps-gated intensified CCD-camera. Spectral and decay data are recorded simultaneously, which makes data acquisition fast compared to time-correlated single-photon counting.

Fast-gated intensified charge-coupled device camera to record time-resolved fluorescence spectra of tryptophan.

The possibilities of a 200 ps gated intensified charge-coupled device (CCD) camera to record time-resolved fluorescence were explored using the fluorescing amino acid tryptophan and its derivative Nacetyl-tryptophan amide (NATA) as model compounds. The results were compared to complementary data from time-correlated single-photon counting (TCSPC) experiments. If a spectral resolution of 1-2 nm is desired, the fast-gated intensified CCD (ICCD) camera is the method of choice.