The 4s ← 1b₁ and 3d ← 1b₁ Rydberg states of H₂O and D₂O: Spectroscopy and predissociation dynamics

Abstract

Two new electronic states of H2O and D2O have been identified in the energy range 84 000–88 000 cm−1 as three-photon resonances in four-photon ionization spectroscopy. Simulations of the rotational intensity distributions using asymmetric top three-photon line strength theory, and rotational analyses, characterize the states as B1 and A2. These Rydberg states are assigned to the excitations 4sa1 ← 1b1[Formula: see text] and 3d2 ← 1b1[Formula: see text] on the basis of equilibrium geometries, quantum defects, and the polarization dependence of their three-photon transition probabilities. The identification of the one-photon forbidden 1A2–1A1 transition, together with published vacuum ultraviolet (VUV) absorption spectra, permits a consistent assignment for all five members of the 3d ← 1b1 complex.The [Formula: see text] and [Formula: see text] states arc predissociatcd via both homogeneous and heterogeneous mechanisms. The homogeneous channel from the [Formula: see text] state shows a dramatic isotope effect, being about two orders of magnitude faster in H2O than from equivalent levels of D2O. The heterogeneous predissociation exhibits irregular vibronic and isotopic dependencies, which can be rationalized in terms of the intercessional role of accidental near resonances with levels of the heavily predissociated [Formula: see text] state. The (000) levels of the [Formula: see text] states of H2O and D2O show contrasting heterogeneous predissociation behaviour, which can be interpreted with a knowledge of the relevant potential energy surfaces and the electronic–rotational Coriolis interactions that couple the states.

Dissociation and predissociation in the ns 1B1 and np1A2 series of H2O

“…Test results for a one dimensional model of the nsa 1 1 B 1 series of H 2 O correctly reproduce an observed [18] dramatic difference between the predissociation linewidths of the 4s(000) 1 B 1 states of H 2 O and D 2 O, as well as the correct width of the 3s continuum absorption profile [30]. An important consequence of the model, for future spectroscopic analysis, is the prediction of much stronger vibrational anharmonicity in the lower ns 1 B 1 states, than in the positive ion, to which the 14 A golden rule perturbation formula, which closely approximated the MQDT linewidths, was used to obtain a clearer physical understanding of the origin of the observed isotope effect.…”

“…One also knows that bentD 1 A 1 andẼ 0 1 B 2 states are strongly predissociated by configuration interaction with the linearB 1 A 1 and 3pb 2 1 B 2 states respectively [15,16]. In addition quantum state selected predissociation rates for the bent 3pa 1C 1 B 1 , 4sD 0 1 B 1 and 3db 2D 00 1 A 2 states have been reported [17,18]. As a first step towards extension of the theory to higher members of the Rydberg series, this paper offers a theory for homogeneous predissociation from the ns 1 B 1 and npb 2 1 A 2 states, which are not covered by the above Renner-Teller coupling and configuration interaction mechanisms.…”

“…Knowledge that the 3s à 1 B 1 state of H 2 O is directly dissociative, with two equivalent H + OH products, while the 4sD 00 1 B 1 state predissociates sufficiently slowly to allow the observation of resolved rotational structure [18], suggests that a typical section through the manifold of potential surfaces, in the direction of the dissociation mode, q, must take the form in Fig 1. Such behavior is found to be consistent with eqn (1) if V + (q) and the quantum defect function µ s (q) are taken in the forms 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 …”

“…The dissociation dynamics of these two states, which includes an important Renner-Teller coupling mechanism [10], have been very fully characterized, both experimentally (see [11]) and theoretically (see [12]). Less attention has been given to the corresponding dynamics of the higher Rydberg states, except to note that similar Renner-Teller coupling between the so far unobserved bent dissociative 3pb 2 ← 1b 1 1 A 2 state and its linear 3pb 2 ← 3a 1 1 B 2 counterpart allows the observation of only the K 0 a = 0 branches of the latter state [13,14] [17,18]. As a first step towards extension of the theory to higher members of the Rydberg series, this paper offers a theory for homogeneous predissociation from the ns 1 B 1 and npb 2 1 A 2 states, which are not covered by the above Renner-Teller coupling and configuration interaction mechanisms.…”

“…Within the spirit of MQDT theory [19], vibrational matrix element of sin πµ(Q) and cos πµ(Q) may then be used to compute the non-adiabatic coupling strength responsible for the predissociation. Since experiments on the 4s state [18] show that the homogeneous linewidth is independent of excitation in the modes, ν 1 and ν 2 , the vibrational coordinates Q in eqn (1) are reduced for simplicity to a single dissociation mode q. The computational method closely follows that used by Jungen [20] and Ross et al [21] to describe the competitive predissociation and autoionization of H 2 , although attention is restricted here to energies below the ionization threshold.…”