The UV photodissociation of pyruvic acid (PA) is studied in molecular beams using time-of-flight (TOF) mass spectroscopy and time-sliced velocity map imaging (VMI) following excitation to the first absorption band (S1 ← S0) at 330–380 nm. CH3CO, HOCO, CO, CH3, and H are detected as photodissociation products. The photofragment yield (PFY) spectrum of the H product is recorded at 350–380 nm in He and Ar carrier gases. The spectrum shows sharp vibrational features reflecting the significant rotational cooling achieved in the molecular beam. It matches well the broad features observed in the room temperature absorption spectrum and indicates that the S1 state lives longer than a picosecond. The origin band of the S1 ← S0 transition is identified at 26 710 cm–1, and progressions in the CH3 and C–C torsional modes are tentatively assigned. Kinetic energy release (KER) and angular distributions of CH3CO, HOCO, CO, CH3, and H fragments indicate that additional photon absorption from S1 to the S2/S3 states is facile and is followed by rapid dissociation to the observed fragments. On the basis of the energetics of the different dissociation pathways and analyses of the observed KER distributions, three-body fragmentation processes are proposed as major contributors to the formation of the observed products.

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Rotational, vibrational, and electronic states of formaldehyde and cis-hydroxymethylene products generated in the photodissociation of the hydroxymethyl radical are investigated by sliced velocity map imaging (SVMI) following excitation of the radical to its 3px and 3pz Rydberg states. SVMI of H and D photofragments is essential in these studies because it allows zooming in on low-velocity regions of the images where small threshold signals can be identified. With CH2OD precursors, formaldehyde and hydroxymethylene products are examined separately by monitoring D and H, respectively. Whereas the main dissociation channels lead to formaldehyde and cis-hydroxymethylene in their ground electronic states, at higher excitation energies the kinetic energy distributions (KEDs) of H and D photofragments exhibit additional small peaks, which are assigned as triplet states of formaldehyde and hydroxymethylene. Results obtained with deuterated isotopologs of CH2OH demonstrate that the yield of the triplet state of formaldehyde decreases upon increasing deuteration, suggesting that the conical intersection seams that govern the dynamics depend on the degree of deuteration. The rotational excitation of cis-hydroxymethylene depends on the excited Rydberg state of CH2OD and is lower in dissociation via the 3pz state than via the lower lying 3px and 3s states. Vibrational excitation of cis-HCOD, which spans the entire allowed internal energy range, consists mostly of the CO-stretch and in-plane bend modes. When the internal energy of cis-HCOD exceeds the dissociation threshold to D + HCO, slow D and H photofragments deriving from secondary dissociation are observed. The yields of these H and D fragments are comparable, and we propose that they are generated via prior isomerization of cis-HCOD to HDCO.

Vertical and adiabatic excitation energies and oscillator strengths for valence and Rydberg states of hydroxycarbene (HCOH) and methylhydroxycarbene (CHCOH) are reported. The electronic properties were computed with equation-of-motion coupled-cluster methods with single and double substitution methods (EOM-CCSD) and the aug-cc-pVTZ basis set. The states' characters were analyzed by plotting natural transition orbitals (NTOs). The calculations demonstrate that the shape, size, and energy of each Rydberg orbital are affected to varying degrees by their interaction with the ion core. Likewise, the corresponding quantum defects reflect the Rydberg electron-ion core interactions. The results reported herein, combined with previously reported calculations of the photoelectron spectrum of HCOH, should help in designing strategies for state-selective detection of hydroxycarbenes via ionization.

The 205-230 nm photodissociation of vibrationally excited CO at temperatures up to 1800 K was studied using Resonance Enhanced Multiphoton Ionization (REMPI) and time-sliced Velocity Map Imaging (VMI). CO molecules seeded in He were heated in an SiC tube attached to a pulsed valve and supersonically expanded to create a molecular beam of rotationally cooled but vibrationally hot CO. Photodissociation was observed from vibrationally excited CO with internal energies up to about 20 000 cm, and CO(XΣ), O(P), and O(D) products were detected by REMPI. The large enhancement in the absorption cross section with increasing CO vibrational excitation made this investigation feasible. The internal energies of heated CO molecules that absorbed 230 nm radiation were estimated from the kinetic energy release (KER) distributions of CO(XΣ) products in v″ = 0. At 230 nm, CO needs to have at least 4000 cm of rovibrational energy to absorb the UV radiation and produce CO(XΣ) + O(P). CO internal energies in excess of 16 000 cm were confirmed by observing O(D) products. It is likely that initial absorption from levels with high bending excitation accesses both the AB and BA states, explaining the nearly isotropic angular distributions of the products. CO(XΣ) product internal energies were estimated from REMPI spectroscopy, and the KER distributions of the CO(XΣ), O(P), and O(D) products were obtained by VMI. The CO product internal energy distributions change with increasing CO temperature, suggesting that more than one dynamical pathway is involved when the internal energy of CO (and the corresponding available energy) increases. The KER distributions of O(D) and O(P) show broad internal energy distributions in the CO(XΣ) cofragment, extending up to the maximum allowed by energy but peaking at low KER values. Although not all the observations can be explained at this time, with the aid of available theoretical studies of CO VUV photodissociation and O + CO recombination, it is proposed that following UV absorption, the two lowest lying triplet states, aB and bA, and the ground electronic state are involved in the dynamical pathways that lead to product formation.