We report a spectroscopic hyperfine structure study of the (YbF)-Yb-174 mol
ecule in the excited states A (2)Pi(1/2) (v=0), A (2)Pi(1/2) (v=1), and [18
.6]0.5(v=0). The fluorine hyperfine interaction constant A(parallel to) in
each state is, respectively, -1.6+/-2 MHz, 48.8+/-3 MHz, and 63.3+/-3 MHz.
The corresponding values of A(perpendicular to) are 4.6+/-1 MHz, -24.7+/-1
MHz, and -31.9+/-1 MHz. The very small hyperfine interaction characteristic
of A (2)Pi(1/2) (v=0) is not seen in A (2)Pi(1/2) (v=1) because this state
is strongly mixed with [18.6]0.5(v=0). Analysis of our results shows that
the lower-lying of the two mixed states is predominantly of [18.6]0.5(v=0)
character, not A (2)Pi(1/2) (v=1) as had previously been thought. The deper
turbed hyperfine constants for [18.6]0.5 are A(parallel to)=114+/-5 MHz and
A(perpendicular to)=-61+/-2 MHz, comparable with those in the X state and
much stronger than in the unperturbed A (2)Pi(1/2) state. This supports the
view that the state [18.6]0.5 is of 4f(13)6s(2) character. (C) 1999 Americ
an Institute of Physics. [S0021-9606(99)00717-5].