THz Time-Domain Spectroscopy on Ammonia

Abstract

We use the technique of terahertz time-domain spectroscopy to experimentally and theoretically study the absorption and the dispersion of ammonia vapor. Our measurements show small deviations from the rotationinversion spectrum which originate from transitions in the ν 2 hot band and from the (NH 3 ) 2 dimer. We investigate the molecular tunneling in ammonia and apply our new molecular response theory to analyze the inversion spectrum of this molecule. The theory includes the molecular response of polar molecules to an external electric field over the duration of a collision with the molecular response time as a control parameter, and it unifies the basic collision theories of Lorentz, van Vleck-Weisskopf, and Debye. The calculated absorption and dispersion based on this theory fits the measurements over the full spectral range of the THz spectrum and allows to determine a response time of the order of 100 fs.

The submillimeter: A spectroscopist’s view

“…It is common for emerging technologies to seek gas phase spectroscopy applications -in many ways the native scientific application for this spectral region -as they develop (e.g. terahertz time domain approaches used gas phase problems as demonstrations [5][6][7][8][9][10][11] before moving onto field such as medical applications and seeing through walls). This review focuses on high resolution, cw spectroscopy in the region bounded by approximately 100 GHz and 3 THz.…”

T-Ray Sensing and Imaging

“…364 T-ray measurements allow absorption, dispersion and line shape data to support theoretical models of these molecules. 77,361,362 Although demonstrating low average power, the ps time scale of the T-ray pulses results in very high peak powers, therefore T-rays can be used for spectroscopy of samples with large average background THz radiation, such as flames. Gases with no permanent dipole moments, such as N2, O2 and CO2, show no T-ray absorption, whereas the concentrations of H2O, CH and NH 3 in flames can be estimated from the T-ray spectra.…”

Molecular Response Theory in Terms of the Uncertainty Principle

“…The theoretical concept applied to analyze THz-TDS measurements of symmetric top molecules has already previously been described [2][3][4] . In THz-TDS two electromagnetic pulse shapes are measured, the input (reference) pulse and the propagated pulse, which has changed shape due to its passage through the sample under study.…”

“…Some first but still restricted insight into this molecular response we could derive by studying the propagation of femtosecond (fs) THz pulses in dense molecular vapors by means of the powerful technique of THz time domain spectroscopy (THz-TDS) [1][2][3][4] . To explain all details of such measurements it was found necessary to expand the classical collision theories of Lorentz 5 In this contribution we present an extended analysis of some previous and also new THz-TDS measurements for the methyl halides, for tri-fluorine methane (CHF 3 ) and for ammonia 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 4 (NH 3 ) with the objective to derive an advanced understanding of the molecular response and transition time after an external excitation.…”