The coupling of the X 1 + and a 3 + states of the atom pair Na + Cs and modelling cold collisions

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The lowest (1)Pi state of the NaCs molecule, the B(1)(1)Pi state, was studied using a dye laser for inducing fluorescence that was resolved by a high resolution Fourier-transform spectrometer. The presence of argon buffer gas yielded rich rotational relaxation spectra allowing to enlarge the data set for the B(1)(1)Pi state, to obtain Lambda-splittings and to reveal numerous local perturbations. 543 weakly perturbed energy levels for rotational quantum numbers from J(')=5 to 168 and vibrational quantum numbers from v(')=0 to 25, which cover about 87% of the potential well depth, were used for a direct pointwise fit of the potential energy curve applying the inverted perturbation approach method. The resulting potential reproduces the term values for v(')=0-7 with an experimental accuracy of about 0.01-0.02 cm(-1), whereas for v(')=8-25 the deviations increase due to the perturbations, going to the order of 1 cm(-1); an extrapolation is made to the dissociation asymptote.

The (3)(1)Pi state of the NaCs molecule was studied by high resolution Fourier-transform spectroscopy. The (3)(1)Pi-->X (1)Sigma(+) laser induced fluorescence was excited by an Ar(+) ion laser or by a single-mode frequency-doubled cw Nd:YAG laser. The presence of argon buffer gas yielded rich rotational relaxation spectra allowing to enlarge the data set for the (3)(1)Pi state term values, as well as to observe Lambda splittings in a wide range of vibrational (v(')) and rotational (J(')) quantum numbers. The data field includes about 820 energy levels of (3)(1)Pi NaCs in the range from v(')=0 to 37 and from J(')=3 to 190, which corresponds to ca. 95% of the potential well depth. Direct fit of the potential energy curve to the level energies is realized using the inverted perturbation approach method; a set of Dunham coefficients is also presented.

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