Abstract
The photochemistry of the water molecule has revealed a wealth of quantum phenomena, which arise from the involvement of several coupled electronic states with very different potential energy surfaces. Most recently, dissociation from single rotational levels of its ^{1}B_{1} state near 124 nm has been attributed to a vibronically coupled decay via the lower -state surface, despite a large vertical energy gap of 2.8 eV. Similar conclusions have been reached for subsequent experimental data for D_{2}O. The present paper presents further experimental data for HOD and for both the H+OD(X) and D+OH(X) products. Unlike the cases for H_{2}O and D_{2}O, the vibrational populations for hydroxyl products do not follow a smooth distribution with v(OH/OD). In particular, for OH there is a clear alternation in population for all the strong peaks, with odd v favoured over even v. These experimental data are analysed using new MRCI+Q calculations, which have been used to generate potential surfaces and associated non-adiabatic matrix elements for transition from the adiabatic -state to lower unbound potential surfaces; and hence, to guide dynamical calculations using time-dependent wavepackets. It is concluded that although there is a minor contribution from the → decay route, the major route follows → ^{1}A_{2} → . This is mediated through two regions of near degeneracy of the elusive ^{1}A_{2} surface with for short bonds ca. 0.8 Å; and between ^{1}A_{2} and with long bonds ≥2 Å, thereby bridging the vertical energy gap. The striking population alternation for the D+OH(X) products is attributed to dynamic symmetry breaking on the ^{1}A_{2} surface.
This work is supported by the Chinese Academy of Sciences, The Ministry of Science and Technology, and the National Science Foundation of China. R.N.D. is supported by a University of Bristol Senior Research Fellowship, and T.A.A.O. by the UK Engineering and Physical Sciences Research Council.
I. INTRODUCTION
II. EXPERIMENTAL METHODS
III. RESULTS AND DISCUSSION
A. HOD photodissociation
B. Potential energy surfaces
C. Non-adiabatic coupling
D. Dynamical calculations
1. H_{2}O
2. D_{2}O
3. HOD
IV. CONCLUSIONS
Key Topics
- Surface states
- 15.0
- Dissociation
- 12.0
- Photodissociation
- 10.0
- Potential energy surfaces
- 8.0
- Rydberg states
- 8.0
Figures
Total kinetic energy release spectra via excitation to the 0_{00} rotational level of the -state of HOD leading (a) to D+OH and (b) to H+OD, with combs indicating vibration assignments of the X^{2}Π products.
Total kinetic energy release spectra via excitation to the 0_{00} rotational level of the -state of HOD leading (a) to D+OH and (b) to H+OD, with combs indicating vibration assignments of the X^{2}Π products.
Vibrational populations of (a) OH(X^{2}Π) and (b) OD(X^{2}Π), following dissociation of HOD excited via low lying rotational levels of its -state.
Vibrational populations of (a) OH(X^{2}Π) and (b) OD(X^{2}Π), following dissociation of HOD excited via low lying rotational levels of its -state.
Potential energy surfaces for (a) ^{1}B_{1}, (b) ^{1}A_{2}, and (c) ^{1}B_{1}, with a fixed ∠HOH = 104.5°. The contour lines are at multiples of 4000 cm^{−1} relative to the potential minimum for ^{1}A_{1} and are labelled in multiples of 1000 cm^{−1}, with an upper cut-off at 100 000 cm^{−1}.
Potential energy surfaces for (a) ^{1}B_{1}, (b) ^{1}A_{2}, and (c) ^{1}B_{1}, with a fixed ∠HOH = 104.5°. The contour lines are at multiples of 4000 cm^{−1} relative to the potential minimum for ^{1}A_{1} and are labelled in multiples of 1000 cm^{−1}, with an upper cut-off at 100 000 cm^{−1}.
The coordinate dependant T _{ mn } coupling matrix elements of (∂/∂r _{1} + ∂/∂r _{2}). Blue and red contour lines are for positive and negative amplitudes, respectively. (a) T _{ 12 } ≡ ∥ ^{1}A_{2}, peaks at ±2.1 Å^{−1}, contour interval 0.05 Å^{−1}. (b) T _{ 13 } ≡ ∥ , peak at +5.2 Å^{−1}, interval 0.05 Å^{−1}, and (c) T _{ 23 } ≡ ^{1}A_{2}∥ , peaks at ±15.5 Å^{−1}, interval 0.1Å^{−1}. The bold features near r _{min} = 0.7 Å are truncation artefacts at the inner edges of the data set, and are masked out for use in the dynamics calculations.
The coordinate dependant T _{ mn } coupling matrix elements of (∂/∂r _{1} + ∂/∂r _{2}). Blue and red contour lines are for positive and negative amplitudes, respectively. (a) T _{ 12 } ≡ ∥ ^{1}A_{2}, peaks at ±2.1 Å^{−1}, contour interval 0.05 Å^{−1}. (b) T _{ 13 } ≡ ∥ , peak at +5.2 Å^{−1}, interval 0.05 Å^{−1}, and (c) T _{ 23 } ≡ ^{1}A_{2}∥ , peaks at ±15.5 Å^{−1}, interval 0.1Å^{−1}. The bold features near r _{min} = 0.7 Å are truncation artefacts at the inner edges of the data set, and are masked out for use in the dynamics calculations.
An apparent avoided crossing of the ^{1}A_{2} and surfaces for r _{ mean } = 1.75 Å as a function of (r _{ 1 } –r _{ 2 }), i.e., the anti-symmetric stretching coordinate. The minimum separation for this value of r _{ mean } is 6500 cm^{−1}.
An apparent avoided crossing of the ^{1}A_{2} and surfaces for r _{ mean } = 1.75 Å as a function of (r _{ 1 } –r _{ 2 }), i.e., the anti-symmetric stretching coordinate. The minimum separation for this value of r _{ mean } is 6500 cm^{−1}.
Experimental and calculated population distributions for OH(X,^{2}Π,v) products from pre-dissociation of (a) the 0_{00} level of the H_{2}O( ) state, and (b) the 0_{00} and 1_{01} levels of the HOD( ) state. The calculated distributions for the → and → ^{1}A_{2} → decay paths are each normalised to a sum of 100, as are the similarly normalised experimental distributions.
Experimental and calculated population distributions for OH(X,^{2}Π,v) products from pre-dissociation of (a) the 0_{00} level of the H_{2}O( ) state, and (b) the 0_{00} and 1_{01} levels of the HOD( ) state. The calculated distributions for the → and → ^{1}A_{2} → decay paths are each normalised to a sum of 100, as are the similarly normalised experimental distributions.
Population distributions for OD(X,^{2}Π,v) products, as in Figure 6 , for the 0_{00} and 1_{01} levels of (a) the D_{2}O( ) state, and (b) the HOD( ) state.
Population distributions for OD(X,^{2}Π,v) products, as in Figure 6 , for the 0_{00} and 1_{01} levels of (a) the D_{2}O( ) state, and (b) the HOD( ) state.
The calculated outgoing wavepacket on the ^{1}A_{2} surface following excitation of HOD, in Jacobi coordinates with R = D–OH_{cm} and r = O–H, superimposed on contour lines for the potential (Fig. 3(b) ).
The calculated outgoing wavepacket on the ^{1}A_{2} surface following excitation of HOD, in Jacobi coordinates with R = D–OH_{cm} and r = O–H, superimposed on contour lines for the potential (Fig. 3(b) ).
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