Fourier Transform Microwave Spectroscopy Measurement and Instrumentation

Abstract

Microwavemolecular spectroscopy is progressing impressively. In part, profiting from recent experimental developments that combine jet‐expansion sources with elaborate means for preparation of new chemical systems, advances in spectroscopic instrumentation — namely, the rise of very sensitive and high‐resolution time‐domain techniques — are even more significant. Precise structural studies have always been a strength of rotational spectroscopy; now problems arising from convoluted intramolecular dynamics, such as conformational, tautomeric equilibria, and other large amplitude motions, as well as intermolecular vibrational energy redistribution and isomerization, can be targeted with the unrivaled precision of rotational investigations. Numerous interesting chemical systems like molecules with multiple concurrent internal motions, larger complexes, aggregates, biomolecules, and transient species were characterized recently — positioning microwave spectroscopy today as a prime method for the investigation of multidimensional flexible systems. Rotational studies elucidate an extensive range of intermolecular interactions, ranging from hydrogen bonding and van der Waals interaction to the unusual effects observed in quantum solvation. The development of microwave pulsed excitation multiresonance techniques and the advent of instantaneous broadband‐coverage microwave spectroscopy are impressively boosting the capabilities of rotational spectroscopy to quite routinely disentangle the unique spectral signatures that encode the structure and dynamics of hitherto pathological molecular cases. The versatile capabilities of microwave coherence spectroscopy are outlined in the treatment of time‐dependent interaction of rotating molecules with radiation.

The Conformational Map of Volatile Anesthetics: Enflurane Revisited

“…The rotational spectrum was measured initially with resonatorbased Balle-Flygare Fourier-transform microwave spectrometers [27,28] (8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23)(24)(25)(26) and later with ac hirped-pulse broadband spectrometer [29] (2-18 GHz), fully described before. In these instruments either am onochromatic or ac hirped pulse are used to excite ac oherent polarization in ag aseous polar sample.…”

“…Recently, a conformational exploration of enflurane in solution was also reported . We address here all these issues exploiting the sub‐Doppler resolution [kHz], isotopologue‐sensitivity and broadband capabilities of modern microwave spectroscopy, supplemented with high‐level ab initio calculations. The results now establish unambiguously the conformational map and molecular structure of enflurane, paving the way for future analysis of enflurane clustering properties with high‐resolution techniques…”

The Hunt for Elusive Molecules: Insights from Joint Theoretical and Experimental Investigations

“…[10] Beyond sensitivity, a key aspect of cavity-enhanced FTMW spectroscopy is that line intensities scale linearly with the dipole moment µ, and not with µ 2 as in conventional absorption rotational spectroscopy. [11] As a consequence, rotational lines of weakly polar molecules are more readily detected by this method compared to either classical emission or absorption spectroscopy. Detection sensitivity is achieved at the expense of instantaneous bandwidth (IB), however, which is limited to about 1 MHz (assuming a pulse length of ∼1 µs), making spectral surveys over hundreds of MHz extremely tedious by manual operation.…”

Edge effects in chirped-pulse Fourier transform microwave spectra

“…13 and reviewed in detail in Ref. 16. If phenomenological dephasing terms are ignored, the optical Bloch equations for this situation is exactly solvable.…”