A study of the predissociation of NaK molecules in the 6 1Σ+ state by optical–optical double resonance spectroscopy

Abstract

Predissociation of a high-lying Σ+1 state of NaK is studied using the optical–optical double resonance technique. A single-mode ring dye laser is set to a particular 2(A)1Σ+(v,J)←1(X)1Σ+(v′′,J′′) transition. Another single-mode laser (Ti–sapphire) is then used to excite the molecule from the 2(A)1Σ+(v,J) level, to rovibrational levels of a higher predissociating electronic state, which we identify as 6 1Σ+. The predissociation is monitored by the atomic potassium emission on the 3 2D3/2→4 2P1/2 transition at 1.17 μm, while bound state radiative processes are monitored by total violet fluorescence from the upper state to the various rovibrational levels of the ground 1(X)1Σ+ state. By scanning the Ti–sapphire laser, different rovibrational levels of the 6 1Σ+ state can be excited. The vibrational levels probed range from v=13 to 20 with rotational states ranging from 9 to 99. The bound state energy level positions are measured from the center frequencies of lines recorded with the Ti–sapphire laser excitation scans. The 6 1Σ+ state is then described by the following molecular constants which are calculated from the experimental values of the level energies: Te=25 560.373 cm−1, ωe=89.179 26 cm−1, ωexe=0.730 691 cm−1, Be=0.067 327 0 cm−1, αe=0.000 675 35 cm−1, De=−3.298 31×10−8 cm−1, βel=1.518 17×10−8 cm−1. The potential well depth is De=4416.0 cm−1, if we assume the most likely asymptotic limit of Na(3 2S1/2)+K(5 2P1/2). The equilibrium separation is Re=4.158 Å. We also report measured and calculated intensities (Franck–Condon factors) for the 6 1Σ+→1(X)1Σ+ violet band. The absolute predissociation rates of 6 1Σ+ levels are directly measured from the linewidths recorded on the Ti–sapphire laser excitation scans. We measure predissociation rates ranging up to 9.4×109 s−1. The dependence of the absolute predissociation rates on rovibrational quantum numbers is studied with an attempt to predict the shape of the repulsive potential curve causing the predissociation, its crossing point with the bound state, and the type of perturbative interaction leading to the predissociation. The state causing the predissociation is determined from correlation diagrams to be the continuum of either the 3 3Π, the 3 1Π, or the 5 3Σ+ state with Na(3S)+K(3D) dissociation limit. We measure the collisional broadening rate coefficients of some 6 1Σ+←2(A)1Σ+ lines due to both argon and potassium perturbers, and obtain the average values, kbrAr=(1.1±0.2)×10−8 cm3 s−1 and kbrK=(1.1±0.6)×10−8 cm3 s−1. Velocity-changing collisions and collisional excitation transfer between individual rotational levels of the 2(A)1Σ+ state are also investigated.