Design and performance of a throughput-matched, zero-geometric-loss, modified three objective multipass matrix system for FTIR spectrometry

The design of and initial results obtained from a multipass matrix system (MMS) for mid-infrared spectroscopy that operates in the Highly Instrumented Reactor for Atmospheric Chemistry (HIRAC) recently constructed in the School of Chemistry at the University of Leeds, is described. HIRAC is an evacuable, temperature variable, photochemical atmospheric reaction chamber. The MMS design is a modified Chernin cell, utilizing three objective mirrors and two field mirrors. In addition to providing the paraxial equations required for design of a throughput matched multipass cell and throughput matched transfer optics, advanced ray tracing simulations have been performed for the Chernin design described herein. The simulations indicate that, for this MMS, which features small off-axis angles and preserves perfectly the focal properties of the original White design, the paraxial equations are nearly exact, throughput losses due to astigmatism are insignificant, and the system has zero theoretical geometric loss. Measurements of the signal incident on the detector at different matrix arrangements confirm the ray trace results, suggesting that geometric loss in this system is insignificant. The MMS described herein provides adequate stability to permit measurements while the chamber mixing fans are on, gives very good detection limits for some representative species, and is easy to align.