A sub-Doppler resolution double resonance molecular beam infrared spectrometer operating at chemically relevant energies (∼2 eV)

Abstract

A molecular beam spectrometer capable of achieving sub-Doppler resolution at 2 eV ͑ϳ18 000 cm Ϫ1 ) of vibrational excitation is described and its performance demonstrated using the CH stretch chromophore of HCN. Two high finesse resonant power-buildup cavities are used to excite the molecules using a sequential double resonance technique. A vϭ0→2 transition is first saturated using a 1.5 m color center laser, whereupon a fraction of the molecules is further excited to the vϭ6 level using an amplitude modulated Ti:Al 2 O 3 laser. The energy absorbed by the molecules is detected downstream of both excitation points by a cryogenically cooled bolometer using phase sensitive detection. A resolution of approximately 15 MHz ͑i.e., three parts in 10 8 ) is demonstrated by recording a rotational line in the vϭ6 manifold of HCN. Scan speeds of up to several cm Ϫ1 /h were obtained, with signal-to-noise ratios in excess of 100. The high signal-to-noise ratio and a dynamic range of 6ϫ10 4 means that future experiments to study statistical intramolecular vibrational energy redistribution in small molecules and unimolecular isomerizations can be attempted. We would also like to point out that, with improved metrology in laser wavelengths, this instrument can also be used to provide improved secondary frequency standards based upon the rovibrational spectra of molecules.

Rovibrational Energy Transfer in the 4νCH Manifold of Acetylene, Viewed by IR−UV Double Resonance Spectroscopy. 5. Detailed Kinetic Model

“…The above investigation 25 revisits our opening theme of significant advances in molecular physics that rely on innovative, high-performance instrumentation; it exemplifies a variety of eigenstate-resolved IR spectroscopic studies of congested polyatomic-molecular rovibrational manifolds, performed with optothermal detection by Lehmann, Scoles, and co-workers. − In particular, we also note their measurements 23 of high-resolution optothermal rovibrational spectra for the ν CH , 2ν CH , and 3ν CH acetylene stretching bands of propyne (CH 3 CCH) and for the 2ν CH band of trifluoropropyne (CF 3 CCH); these provide interesting contrasts with the lighter, supposedly simpler (but apparently not much less complicated!) C 2 H 2 molecule in its 4ν CH region, on which the present paper has focused.…”

“…There are numerous instances in molecular physics of significant scientific discoveries being preceded by courageous engineering projects that entail the building of new, high-performance instruments, taking advantage of emerging technology and almost invariably driven by a visionary determination to make measurements that surpass what were previously feasible. A prime example in this context is the emergence of optothermal molecular-beam spectroscopy, pioneered 30 years ago by Gough, Miller, and Scoles. , Several key technological elements were involved in this development: a liquid-helium-cooled doped silicon superconducting bolometer , which operated at ∼2 K and was of a type previously used to measure molecular beam scattering; continuous-wave single-longitudinal-mode F-center lasers , that were continuously tunable in the near-infrared region, with new-found reliability; a profound insight into high-vacuum technology, enabling essential instrumental engineering to be optimized. ,− (In this last respect, Scoles has lauded “the superiority of common sense over complicated thinking,” as exemplified in J. B. Fenn's approach to such research.)…”

Rovibrational spectroscopy of the v=6 manifold in C212H2 and C213H2

“…27 In brief, a molecular beam is formed by expanding a 1% mixture of acetylene in helium through a 30 m nozzle. The expansion is collimated by a 275 m conical skimmer located approximately 1 cm downstream, which extracts the centerline portion of the expansion and isolates the high pressure source chamber (10 Ϫ4 torr) from the low pressure detector chamber (10 Ϫ7 torr).…”

AC Stark Effect Observed in a Microwave–Millimeter/Submillimeter Wave Double-Resonance Experiment

“…Many experiments utilizing double resonance have been conducted across a wide range of wavelengths, which include radio−microwave, 2,3 microwave−microwave, 4−10 microwave− millimeter, 11−15 microwave−infrared, 16−26 microwave−optical, 27−32 microwave−ultraviolet, 33,34 microwave−X-ray, 35 millimeter−infrared, 36 millimeter−optical, 37−40 infrared−infrared, 41−43 infrared−optical, 44 infrared−ultraviolet, 45−49 infrared−X-ray, 50 optical−optical, 51−56 optical−ultraviolet, 57 ultraviolet−ultraviolet, 58,59 ultraviolet−X-ray, 60 and phosphorescence−microwave. 61,62 Of these previously conducted experiments, it is most relevant to focus on the five previous works reporting microwave−millimeter double resonance.…”

“…Many experiments utilizing double resonance have been conducted across a wide range of wavelengths, which include radio–microwave, , microwave–microwave, − microwave–millimeter, − microwave–infrared, − microwave–optical, − microwave–ultraviolet, , microwave–X-ray, millimeter–infrared, millimeter–optical, − infrared–infrared, − infrared–optical, infrared–ultraviolet, − infrared–X-ray, optical–optical, − optical–ultraviolet, ultraviolet–ultraviolet, , ultraviolet–X-ray, and phosphorescence–microwave. , Of these previously conducted experiments, it is most relevant to focus on the five previous works reporting microwave–millimeter double resonance. Three of these experiments were conducted by Endo and co-workers, ,, while the other two were conducted by Jäger and co-workers. , These double-resonance experiments were conducted with point-by-point scans, instead of using a chirp or a sweep modulation to achieve broadband coverage.…”