We describe an instrument which can be used to analyze complex chemical mixtures at high resolution and high sensitivity. Molecules are collisionally cooled with helium gas at cryogenic temperatures (∼4–7 K) and subsequently detected using chirped pulse microwave spectroscopy. Here, we demonstrate three significant improvements to the apparatus relative to an earlier version: (1) extension of its operating range by more than a factor of two, from 12–18 GHz to 12–26 GHz, which allows a much wider range of species to be characterized; (2) improved detection sensitivity owing to the use of cryogenically cooled low-noise amplifiers and protection switches; and (3) a versatile method of sample input that enables analysis of solids, liquids, gases, and solutions, without the need for chemical separation (as demonstrated with a 12–16 GHz spectrum of lemon oil). This instrument can record broadband microwave spectra at comparable sensitivity to high Q cavity spectrometers which use pulsed supersonic jets, but up to 3000 times faster with a modest increase in the sample consumption rate.

Polyatomic molecular ions contain many desirable attributes of a useful quantum system, including rich internal degrees of freedom and highly controllable coupling to the environment. To date, the vast majority of state-specific experimental work on molecular ions has concentrated on diatomic species. The ability to prepare and read out polyatomic molecules in single quantum states would enable diverse experimental avenues not available with diatomics, including new applications in precision measurement, sensitive chemical and chiral analysis at the single-molecule level, and precise studies of Hz-level molecular tunneling dynamics. While cooling the motional state of a polyatomic ion via sympathetic cooling with a laser-cooled atomic ion is straightforward, coupling this motional state to the internal state of the molecule has proven challenging. Here we propose a method for readout and projective measurement of the internal state of a trapped polyatomic ion. The method exploits the rich manifold of technically accessible rotational states in the molecule to realize robust state preparation and readout with far less stringent engineering than quantum logic methods recently demonstrated on diatomic molecules. The method can be applied to any reasonably small $(\lesssim 10$ atoms) polyatomic ion with an anisotropic polarizability.

Spectroscopy is a key analytical tool that provides valuable insight into molecular structure and is widely used to identify chemical samples. Tagging spectroscopy is a form of action spectroscopy in which the absorption of a single photon by a molecular ion is detected via the loss of a weakly attached, inert ‘tag’ particle (for example, He, Ne, N₂)^1–3. The absorption spectrum is derived from the tag loss rate as a function of incident radiation frequency. So far, all spectroscopy of gas phase polyatomic molecules has been restricted to large molecular ensembles, thus complicating spectral interpretation by the presence of multiple chemical and isomeric species. Here we present a novel tagging spectroscopic scheme to analyse the purest possible sample: a single gas phase molecule. We demonstrate this technique with the measurement of the infrared spectrum of a single gas phase tropylium (C₇H₇⁺) molecular ion. The high sensitivity of our method revealed spectral features not previously observed using traditional tagging methods⁴. Our approach, in principle, enables analysis of multicomponent mixtures by identifying constituent molecules one at a time. Single molecule sensitivity extends action spectroscopy to rare samples, such as those of extraterrestrial origin^5,6, or to reactive reaction intermediates formed at number densities that are too low for traditional action methods.

We present a new algorithm, Robust Automated Assignment of Rigid Rotors (RAARR), for assigning rotational spectra of asymmetric tops. The RAARR algorithm can automatically assign experimental spectra under a broad range of conditions, including spectra comprised of multiple mixture components, in ≲100 s. The RAARR algorithm exploits constraints placed by the conservation of energy to find sets of connected lines in an unassigned spectrum. The highly constrained structure of these sets eliminates all but a handful of plausible assignments for a given set, greatly reducing the number of potential assignments that must be evaluated. We successfully apply our algorithm to automatically assign 15 experimental spectra, including 5 previously unassigned species, without prior estimation of molecular rotational constants. In 9 of the 15 cases, the RAARR algorithm successfully assigns two or more mixture components.

Background : Workers who sprayed phenoxy acid herbicides, especially those who sprayed before 1975, may have been exposed to significant amounts of 2, 3, 7, 8-tetrachlorodi-benzo- p -dioxin (TCDD), a potent animal carcinogen present in herbicide preparations as a contaminant. Purpose : The aims of this study were ( a ) to determine serum levels of TCDD in a representative sample of workers occupationally exposed to the agent during the spraying of phenoxy acid herbicides; ( b ) to compare serum levels in workers exposed before 1965, when concentrations in herbicide products were unregulated and high, with levels in workers exposed after 1974, when concentrations were lower as a result of government regulations worldwide; and ( c ) to examine the correlation, if any, between serum levels and duration of employment in spraying. Methods : Thirty-seven subjects were randomly selected from a group of 654 men who had sprayed the herbicides 2, 4, 5-trichlorophen-oxyacetic acid (2, 4, 5-T) and 2, 4-dichlorophenoxyacetic acid (2, 4-D) in Australia for at least 12 months. The workers were classified as follows: eight who sprayed only before 1965, nine who sprayed only during the period after 1964 and before 1975, and 20 who sprayed during the period after 1974 and before 1991. Serum from the workers was analyzed for TCDD by high-resolution gas chro-matography and high-resolution mass spectrometry at a detection limit of 0.6 parts per trillion (ppt) on a lipid-weight basis. In addition, rates of exposure to TCDD were estimated, as were TCDD serum concentrations at termination of employment and intensity of herbicide use. Results : Only one worker, with a serum TCDD level of 34 ppt, had a serum level higher than the maximum level of 26 ppt reported for the general population. As suming a half-life of 7.1 years, we estimated the mean exposure rates to be 2.7, 2.3, and 0.06 ppt/mo for the three epochs, respectively. We found the highest serum level of TCDD at the time of cessation of employment to be 329 ppt. Calendar period and intensity of use of 2, 4, 5-T and 2, 4-D were statistically significant determinants of rate of exposure to TCDD, but 2, 4-D was associated with exposure rate only for the pre-1975 periods. Estimated rates prior to 1965 were more than an order of magnitude higher than those after 1974. Conclusion : The highest estimated exposure rate was 20.7 ppt/mo, which suggests that some sprayers may have been exposed to levels comparable with those that produce cancer in laboratory animals. [J Natl Cancer Inst 84: 1648–1653, 1992]