Cell multipass

Experimental limitations initially limited the types of molecular systems that could be studied by TRIR spectroscopy. The main obstacles were the lack of readily tunable intense IR sources and sensitive fast IR detectors. Early TRIR work focused on gas phase studies because long pathlengths and/or multipass cells could be used without interference from solvent IR bands. Pimentel and co-workers first developed a rapid scan dispersive IR spectrometer (using a carbon arc broadband IR source) with time and spectral resolution on the order of 10 ps and 1 cm , respectively, and reported the gas phase IR spectra of a number of fundamental organic intermediates (e.g., CH3, CD3, and Cp2). Subsequent gas phase approaches with improved time and spectral resolution took advantage of pulsed IR sources. 

Differential absorption spectroscopy techniques were used to determine absolute species concentrations (C2H4, CO, and CO2) by tuning the wavelength of the lasers across transitions near 1646 nm (i/i +1 9, t 5 + < 9 bands of C2H4), the i 13 transition of CO (3z/ band) near 1564 nm, and the R16 transition of CO2 near 1572 nm 2ui + 2v2 + V9 band). The absorption measurements were recorded in the multipass cell. 

A Nd YAG Laser Multipass Cell for Pulsed Raman-Scattering Diagnostics... 

Optical multipass cells have been used for the enhancement of CW Raman scattering(4) however, these cells are typically not well-suited for use with high power, pulsed lasers. A new multipass cell for use with a pulsed Nd YAG laser is proposed whereby the 1.06 micron laser output is admitted into a multipass cell cavity where it is partially converted to 532nm with a Brewster s angle cut second harmonic generating crystal The 532nm pulse is trapped in the mirrored cavity while the 1.06 micron pulse is dumped. This multipass cell concept has been demonstrated with the experimental set-up shown in figure 1. 

The pump laser is a Quanta-Ray Nd YAG Model DCR-1A with an 8 nsec, 700 mj (max), 1.06 micron output. The multipass cell cavity is bounded by the normal incidence harmonic beamsplitter (>99.5%... 

Nitrogen Stokes vibrational Raman signal from pulsed multipass cell (5)... 

The performance of this pulsed multipass cell is shown in figure 2 where it is seen that the nitrogen vibration Raman multipass signal decays to 10% of its initial strength in 550 nsec or 100 passes. This corresponds to a multipass cell efficiency of 97.7% and a gain of 42. 

At certain positions in the flame, the background flame luminescence received by the photomultiplier tube can be 15% of the Rayleigh scattered intensity. A large reduction of this noise would be achieved by replacing the 1 nm bandpass filter with a monochrometer. Use of a multipass cell (12) or intracavity laser (13) would raise the signal above the flame luminescence. In addition, the increased scattered photon count rate would increase the precision of each measurement. 

Aim multipass cell for i.r. spectroscopy, utilizing two parallel concave mirrors, has allowed path lengths up to 150 m to be achieved. For a study of collision-induced simultaneous transitions in binary gas mixtures, a 2m sample cell has been constructed that allows pressure variation up to 1500 bar. A cell has been designed for pressures up to lOKbar and temperature variation over the range 10—300 A Pfund-type cell has been constructed for i.r. spectroscopy with... 

Unfortunately, experimental limitations have severely restricted types of molecular systems that could be studied by TRIR spectroscopy in the past. The main obstacles have been the lack of readily tunable intense IR sources and sensitive fast IR detectors. Early TRIR work focused on gas phase studies because long path lengths and/or multipass cells [3] could be used without inter-... 

The first step towards higher sensitivities is the increase of the absorption path length L. This can be for instance achieved when the absorption cell is placed inside a multipass cell. One example is shown in Fig. 1.2. Two spherical mirrors with a high reflectivity R are separated by a distance d which nearly equals the mirror radius r forming a nearly spherical resonator (Vol. 1, Sect. 5.2). 

Fig. 1.28 Acoustic resonance cell inside an optical multipass cell. Dimensions are in millimeters...

Fig. 10.1 Multipass cell of an optoacoustic spectrometer. All laser beams pass through regions in the acoustic resonator where the radial acoustic resonance has maximum amplitude. Measurements are in millimeter...

Fig. 10.48 Multipass cell and spectrally selective detector arrangement for the sensitive Raman spectroscopy and diagnostics of molecular gases [1546]...

Figure 1 Examples of gas cells for mid-infrared transmission measurements (A) photograph of a multiple-pass gas cell ( Long Path Miniceir), with a high path-to-volume (530 ml) ratio. Allows paths from 1.2 m (eight passes) to 7.2 m (48 passes) (B) schematic of a multipass cell with transfer optics for use in a center-focus sample compartment (C) and (D) photographs of Pyrex and stainless steel bodied 10-cm pathlength cells, respectively. ((A and B) Reproduced by kind permission of Infrared Analysis, Inc., Anaheim CA, USA. (C and D) Reproduced by kind permission of Specac Ltd., Orpington, Kent, UK.)...

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