High-resolution, broadly-tunable mid-IR spectroscopy using a continuous wave optical parametric oscillator.

We report on the design and automation of a mid-infrared, continuous wave, singly-resonant optical parametric oscillator. Hands-free controls and the implementation of a tuning algorithm allowed for hundreds of nanometers of continuous, effective-mode-hop-free tuning over the range of 2190-4000 nm. To demonstrate the applicability of this light source and algorithm to mid-IR spectroscopy, we performed a sample spectroscopy measurement in a C2H2 gas cell and compared the experimentally-measured absorption spectrum to HITRAN 2016 simulations. We found excellent agreement with simulation in both peak heights and peak centers; we also report a reduced uncertainty in peak centers compared to simulation.

Observation and identification of metastable excited states in ultrafast laser-ionized pyridine.

We report on the fragmentation of ionized pyridine (C(5)H(5)N) molecules by focused 50 fs, 800 nm laser pulses. Such ionization produces several metastable ionic states that fragment within the field-free drift region of a reflectron-type time of flight mass spectrometer, with one particular metastable dissociation being the leading fragmentation process. Because the time of flight is no longer dependent in a simple way on the mass of the ion, the metastable decay is manifested as an unfocused peak on the mass spectrum that appears at a time of flight not corresponding to an integer mass. A previously-developed method is used to identify the precursor and final masses of these ions. The metastable process that creates the most prevalent peak is shown to be C(5)H(5)N(+) → C(4)H(4)(+) + HCN. Simulations confirm this result and place restrictions on the processes for several other observed metastable reactions.

Ultrafast resonance-enhanced multiphoton ionization in the azabenzenes: pyridine, pyridazine, pyrimidine, and pyrazine.

We report on the ultrafast photoionization of pyridine, pyridazine, pyrimidine, and pyrazine. These four molecules represent a systematic series of perturbations into the structure of a benzene ring which explores the substitution of a C-H entity with a nitrogen atom, creating a heterocyclic structure. Data are recorded under intense-field, single-molecule conditions. The pulses (50 fs, 800 nm) are focused into the molecular vapor, and ion mass spectra are recorded for intensities of ~10(13) W/cm(2) to ~10(15) W/cm(2). We measure ion yields in the absence of the focal volume effect without the need for deconvolution of the data. For all targets, stable singly- and doubly-charged parent ions (C(6-n)H(6-n)N(n)(+(+))) are observed with features suggesting resonance-enhanced ionization. From the intensity dependence of the ion yield, we infer that excitation occurs both through (1)ππ* transitions (remnants of the benzene structure) and through (1)nπ* transitions, the latter being a result of Rydberg-like excitations of the lone pair electrons of the nitrogen atoms. Stability against intense-field fragmentation is also discussed.

Ultrafast REMPI in benzene and the monohalobenzenes without the focal volume effect.

We report on the photoionization and photofragmentation of benzene (C(6)H(6)) and of the monohalobenzenes C(6)H(5)-X (X = F, Cl, Br, I) under intense-field, single-molecule conditions. We focus 50-fs, 804-nm pulses from a Ti:sapphire laser source, and record ion mass spectra as a function of intensity in the range ∼10(13) W/cm(2) to ∼10(15) W/cm(2). We count ions that were created in the central, most intense part of the focal area; ions from other regions are rejected. For all targets, stable parent ions (C(6)H(5)X(+)) are observed. Our data is consistent with resonance-enhanced multiphoton ionization (REMPI) involving the neutral (1)ππ* excited state (primarily a phenyl excitation): all of our plots of parent ion yield versus intensity display a kink when this excitation saturates. From the intensity dependence of the ion yield we infer that both the HOMO and the HOMO-1 contribute to ionization in C(6)H(5)F and C(6)H(5)Cl. The proportion of phenyl (C(6)H(5)) fragments in the mass spectra increases in the order X = F, Cl, Br, I. We ascribe these substituent-dependent observations to the different lifetimes of the C(6)H(5)X (1)ππ* states. In X = I the heavy-atom effect leads to ultrafast intersystem crossing to a dissociative (3)nσ* state. This breaks the C-I bond in an early stage of the ultrashort pulse, which explains the abundance of fragments that we find in the iodobenzene mass spectrum. For the lighter X = F, Cl, and Br this dissociation is much slower, which explains the lesser degree of fragmentation observed for these three molecules.