Standards of seven HAQs were purchased from Sigma-Aldrich and used without further purification. Standards were directly applied to individual graphite sample bars as a thin solid layer and examined in isolation to ensure spectral purity for 2C-R2PI and pump-probe measurements.

The instrument has been previously described in detail, and only a brief description of the experimental setup follows (31, 32). Samples were laser desorbed in vacuo directly in front of a pulsed molecular beam controlled by a piezo cantilever valve (33). The desorption laser was a tightly focused Nd:YAG laser (1064 nm, ~1 mJ/pulse), and the piezo cantilever valve operates at a 45-μs pulse duration with eight bars backing argon gas. The desorbed sample was adiabatically cooled by collisions with the argon jet expansion to between 10 and 20 K, and the molecular beam was skimmed before being intersected by laser beams and photoionized by 2C-R2PI. The subsequent ions were detected by a reflectron time of flight mass spectrometer (analyzer pressure, 2 × 10−6 torr; mass resolution, mm = 500).

The 2C-R2PI spectroscopic and picosecond pump-probe delay measurements were performed with an Ekspla PL2251 Nd:YAG laser system producing ∼30-ps laser pulses. The 355-nm output pumps an Ekspla PG401 tunable optical parametric generator (OPG) (output of 450 to 600 μJ/pulse and spectral linewidth of ∼6 cm−1). The sample was excited by the OPG and ionized by 213-nm fifth harmonic of the Ekspla PL2251 laser, which was mechanically delayed up to 600 ps before collineation with the OPG beam. A variable electronic (SRS DG645) delay between OPG UV laser and an excimer laser (193 nm, 1.5 to 2 mJ/pulse) was used for pump-probe measurements in the nanosecond time delay range.

For IR-UV double resonant spectroscopy (i.e., IR hole burning) a Laser Vision optical parametric oscillator/amplifier (mid-IR output over the range 3000 to 3600 cm−1 of ~3 to 5 mJ/pulse and spectral linewidth of 3 cm−1) precedes the R2PI pulse by 200 ns. This study used double resonant spectroscopy with two different pulse sequences: In mode I, the IR pump was scanned at a fixed UV probe wavelength, while in mode II, the UV was scanned with a fixed IR burn wavelength. In mode I, the UV laser wavelength was selected to correspond to a single vibronic transition, and the resulting 2C-R2PI signal depletes when the IR laser becomes resonant with the ground-state population. The resulting ion-dip spectrum therefore represents the ground-state IR spectrum of a single tautomer selected by the UV probe wavelength. This IR spectrum can be compared with calculated IR frequencies to determine the specific tautomer of the selected vibronic transition. In mode II, the IR laser wavelength was selected to correspond to a tautomer-specific vibrational resonance, and spectra were collected both with IR laser on and off. The difference spectrum identifies peaks in the UV spectrum that arise from the same tautomer.

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