Ultra-sensitive high-precision spectroscopy of a fast molecular ion beam

Abstract

Direct spectroscopy of a fast molecular ion beam offers many advantages over competing techniques, including the generality of the approach to any molecular ion, the complete elimination of spectral confusion due to neutral molecules, and the mass identification of individual spectral lines. The major challenge is the intrinsic weakness of absorption or dispersion signals resulting from the relatively low number density of ions in the beam. Direct spectroscopy of an ion beam was pioneered by Saykally and co-workers in the late 1980s, but has not been attempted since that time. Here, we present the design and construction of an ion beam spectrometer with several improvements over the Saykally design. The ion beam and its characterization have been improved by adopting recent advances in electrostatic optics, along with a time-of-flight mass spectrometer that can be used simultaneously with optical spectroscopy. As a proof of concept, a noise-immune cavity-enhanced optical heterodyne molecular spectroscopy (NICE-OHMS) setup with a noise equivalent absorption of ∼2 × 10 −11 cm −1 Hz −1/2 has been used to observe several transitions of the Meinel 1-0 band of N + 2 with linewidths of ∼120 MHz. An optical frequency comb has been used for absolute frequency calibration of transition frequencies to within ∼8 MHz. This work represents the first direct spectroscopy of an electronic transition in an ion beam, and also represents a major step toward the development of routine infrared spectroscopy of rotationally cooled molecular ions.

Doppler broadened NICE-OHMS beyond the triplet formalism: assessment of optimum modulation index

“…It was therefore subtracted from the measured signals. [12,[20][21][22], to the authors' knowledge, these are the first demonstrations of Db-NICE-OHMS signals for β exceeding unity. The corresponding fits of Eq.…”

Vibronic Spectroscopy of Sympathetically Cooled CaH+

“…However, the spectroscopy vital to the detection of this molecular ion has been absent in contrast to other neutral metal hydrides. Short storage times and weak absorption and dispersion signals [17] pose intrinsic challenges to molecular beam spectroscopy, a limitation that has contributed to the lack of experimental data on CaH + . Within the last few decades, new trapping and laser-cooling techniques have made it possible to generate metal hydride ions with quantum state-controlled chemical reactions between laser-cooled trapped atomic ions and molecular hydrogen [18,19,20].…”

Broadband velocity modulation spectroscopy of HfF+: Towards a measurement of the electron electric dipole moment

“…Many other carbocations are interesting for both their presence as intermediates in reactions such as combustion as well as the challenges they present to theory; for example, the spectrum of CH + 5 , a highly nonclassical carbocation, still remains unassigned due to both its complexity and spectral interference from other contaminant species [32,33,34]. A combination of comb-vms with a recently developed ion-beam spectrometer capable of sensitive, sub-Doppler vms of rotationally cold species ( [35]) could enable mass-selective spectroscopy of ions over a broad bandwidth with many potential applications.…”