Hollow cathode ion lasers for deep ultraviolet Raman spectroscopy and fluorescence imaging

Abstract

This article describes the development of hollow cathode ion lasers and their use in constructing an ultraviolet micro-Raman spectrograph with native fluorescence imaging capability. Excitation at 224.3 nm is provided by a helium-silver hollow cathode metal ion laser and at 248.6 nm by a neon-copper hollow cathode metal ion laser. Refractive microscope objectives focus chopped continuous wave laser light on a sample and collect 180°scattered photons. Imaging is accomplished by broadband visible illumination and by deep ultraviolet laser induced excitation of visible wavelength native fluorescence in untagged microorganisms. This makes possible a detection strategy employing rapid imaging with laser excitation to locate regions of native fluorescence activity, followed by deep ultraviolet resonance Raman spectroscopy of the identified fluorescent sites. We have employed this probe for in situ detection of microorganisms on mineral and soil substrates. We present here the deep ultraviolet resonance Raman spectra for the gram negative iron reducing bacterium Shewanella oneidensis obtained while the microorganism remains in situ on the unpolished surface of the mineral calcite and in a Mars soil analog, JSC1. In the current configuration the in situ mineral surface limit of detection for fluorescence is one organism in 2ϫ10 4 m 2 field of view and of order 20-30 microorganisms for Raman spectra. For the Mars soil sample analog fluorescent target selection gives an effective ultraviolet resonance Raman spectral detection limit of 6ϫ10 4 cells/gm or ϳ60 ppb.

Raman technology for future planetary missions

“…The quasi-cw operation is a compromise between pure pulsed operation (laser damage of the sample and potentially the fiber optics) and cw-operation (large power consumption and active cooling required). These compact quasi-cw lasers allow operation without cooling over a large operational temperature range and the power consumption is compatible with an instrument for planetary exploration [4]. Raman measurements are reported with these lasers [5].…”

“…Hans Thiele (1) , Stefan Hofer (1) , Timo Stuffler (1) , Markus Glier (1) , Jürgen Popp (2,4) , Omar Sqalli (3) , Andreas Wuttig (4) , Rainer Riesenberg (4) , (1) …”

“…The instrument as shown in Fig. 2 can be divided in two mayor subsystems, the actual Raman optical system containing laser (1), fiber coupling (2), spectrometer (3) and optical head (4), and the scanning microscope subsystem consisting of precision 3-D actuators (5), which carry the optical head and the optical auto-focus feedback system (6). The scanning microscope system allows to position the optical head above the sample, perform a topography scan and to position the optical head with high accuracy for Raman point measurements or for 2-D mapping measurements.…”

Understanding the Application of Raman Spectroscopy to the Detection of Traces of Life

“…Therefore, the information content of the spectra is high, by comparison with that available from Stokes (conventional) Raman scattering and IR spectroscopy. Resonance Raman spectroscopy is useful for investigating molecules such as RNA, DNA, amino acids, proteins (e.g., Storrie-Lombardi et al, 2001;Tarcea et al, 2007), carotenoids (e.g., Marshall et al, 2007b), and chlorophyll, all of which undergo electronic transitions when excited by UV or visible electromagnetic radiation.…”

Raman Spectroscopy—A Powerful Tool for in situ Planetary Science

“…A promising solution are the so-called hollow cathode NeCu ion-lasers, which emit @ 248.6 nm; these were recently released by Photon Systems. The laser was developed partially with regard to compact science instruments for NASA technology programs (Storrie-Lombardi et al 2001). …”