Predissociation of Rydberg states of CO investigated by the detection of atomic fragments

Vacuum Ultraviolet Photodissociation Branching Ratios of 12C16O, 13C16O, and 12C18O from 100500 to 102320 cm-1

The C⁺ ion photofragment spectra and photodissociation branching ratios into the two energetically available channels, C(¹D) + O(³P) and C(³P) + O(³P), have been obtained for the three CO isotopologues, ¹²C¹⁶O, ¹³C¹⁶O, and ¹²C¹⁸O, in the vacuum ultraviolet range 100500-102320 cm^-1. The two vibronic states of ¹Σ⁺ symmetry, F(3dσ) ¹Σ⁺(υ' = 1) and J(4sσ) ¹Σ⁺(υ' = 0), predominantly dissociate into the lowest channel C(³P) + O(³P) through interactions with the repulsive D'¹Σ⁺ state. All three vibronic states of ¹Π symmetry, E'¹Π(υ' = 1, 2) and G(3dπ) ¹Π(υ' = 0), dissociate into both of the channels above. The photodissociation branching ratios into the channel C(¹D) + O(³P) for E'¹Π(υ' = 1, 2) are found to be independent of both the rotational quantum number and e/f parity, while those for G(3dπ) ¹Π(υ' = 0) strongly depend on the rotational quantum number, indicating very different predissociation pathways between the valence states E'¹Π(υ' = 1, 2) and the Rydberg state G(3dπ) ¹Π(υ' = 0). The potential energy curves of CO in the aforementioned energy range and below have recently been well constructed due to a series of interplays between high-resolution spectroscopic studies and theoretical calculations; the photodissociation branching ratios measured in this study can provide further benchmarks for future theoretical investigations which aim to understand the detailed predissociation dynamics of CO.

Rotational dependence of the branching ratios and fragment angular distributions for the photodissociation of 12C16O in the Rydberg 4p(2) and 5p(0) complex region (92.84-93.37 nm)

¹²C¹⁶O shows complicated absorption structures in the wavelength range of 92.84-93.37 nm caused by the mutual interactions among the Rydberg 4p(2) and 5p(0) complexes, and several diffuse valence states. Here, we systematically measured the branching ratios and fragment angular distributions for the photodissociation of ¹²C¹⁶O in the wavelength range using our mini-time-slice velocity-map imaging (mini-TSVMI) setup and a tunable vacuum ultraviolet (VUV) laser radiation source generated by the two-photon resonance-enhanced four-wave mixing scheme. Various patterns of rotational dependence for the photodissociation branching ratios have been observed for different vibronic states, and they are found to be consistent with previous spectroscopic investigations, revealing the complicated coupling schemes and predissociation dynamics in this region. Irregular angular distributions of the photofragments have been observed, especially in the Rydberg 5p(0) complex region. This has been attributed to the simultaneous excitation of multiple states with different symmetries in this region. The newly observed underlying continuum in the Rydberg 5p(0) complex region has been directly detected in the present study, and it is found to be of ¹Σ⁺ symmetry and dissociates predominantly into the C(³P) + O(³P) channel. This study generally confirms the results of previous spectroscopic studies from a different perspective, and adds new knowledge for understanding the complicated predissociation dynamics of ¹²C¹⁶O in the titled wavelength range.

Isotopic dependence of the predissociations of the E(1)Π state of CO

The predissociations of the E(1)Π state of CO are again studied. They include both the background predissociation attributed to the continuum of the A(1)Π state and the accidental predissociation due to the k(3)Π state. They are calculated using a coupled equations method. The three components of the k state are introduced. These predissociations are studied for different isotopologues and are shown to decrease with increasing reduced mass, in agreement with the experimental results of Ubachs et al. [J. Chem. Phys. 113, 547 (2000)].

Branching ratio measurements for vacuum ultraviolet photodissociation of 12C16O

The branching ratios for the spin-forbidden photodissociation channels of (12)C(16)O in the vacuum ultraviolet (VUV) photon energy region from 102,500 (12.709 eV) to 106,300 cm(-1) (13.180 eV) have been investigated using the VUV laser time-slice velocity-map imaging photoion technique. The excitations to three (1)Σ(+) and six (1)Π Rydberg-type states, including the progression of W(3sσ) (1)Π(v' = 0, 1, and 2) vibrational levels of CO, have been identified and investigated. The branching ratios for the product channels C((3)P) + O((3)P), C((1)D) + O((3)P), and C((3)P) + O((1)D) of these predissociative states are found to depend on the electronic, vibrational, and rotational states of CO being excited. Rotation and e/f-symmetry dependences of the branching ratios into the spin-forbidden channels have been confirmed for several of the (1)Π states, which can be explained using the heterogeneous interaction with the repulsive D'(1)Σ(+) state. The percentage of the photodissociation into the spin-forbidden channels is found to increase with increasing the rotational quantum number for the K(4pσ) (1)Σ(+) (v' = 0) state. This has been rationalized using a (1)Σ(+) to (1)Π to (3)Π coupling scheme, where the final (3)Π state is a repulsive valence state correlating to the spin-forbidden channel.

Branching ratio measurements of the predissociation of 12C16O by time-slice velocity-map ion imaging in the energy region from 108,000 to 110,500 cm(-1)

Direct branching ratio measurements of the three lowest dissociation channels of (12)C(16)O that produce C((3)P) + O((3)P), C((1)D) + O((3)P), and C((3)P) + O((1)D) are reported in the vacuum ultraviolet region from 108,000 cm(-1) (92.59 nm) to 110,500 cm(-1) (90.50 nm) using the time-slice velocity-map ion imaging and nonlinear resonant four-wave mixing techniques. Rotationally, resolved carbon ion yield spectra for both (1)Σ(+) and (1)Π bands of CO in this region have been obtained. Our measurements using this technique show that the branching ratio in this energy region, especially the relative percentages of the two spin-forbidden channels, is strongly dependent on the particular electronic and vibrational energy levels of CO that are excited.

Communication: branching ratio measurements in the predissociation of 12C16O by time-slice velocity-map ion imaging in the vacuum ultraviolet region

The first direct branching ratio measurement of the three lowest energy dissociation channels of CO that produce C((3)P) + O((3)P), C((1)D) + O((3)P), and C((3)P) + O((1)D) is reported. Rotational resolved carbon ion yield spectra for two Π bands (W(3sσ)(1)Π (v(') = 3) at 108,012.6 cm(-1) and (1)Π(v(') = 2) at 109,017 cm(-1)) and two Σ bands ((4sσ)(1)Σ(+)(v(') = 4) at 109,452 cm(-1) and (4pσ)(1)Σ(+)(v(') = 3) at 109,485 cm(-1)) of CO were obtained. Our measurements show that the branching ratio in this energy region is strongly dependent on the electronic and vibrational energy but it is independent or just weakly dependent on the parity and rotational energy levels. To our knowledge, this is the first time that the triplet channel producing O((1)D) has been experimentally observed and this is also the first time that a direct measurement of the branching ratio for the different channels in the predissociation of CO in this energy region has been made.