  We present precise relative radial velocity (RV) measurements for the rapidly
oscillating Ap (roAp) star 33 Librae measured from high resolution data
spanning the wavelength interval 5000--6200 A. We find that pulsational radial
velocity amplitude determined over a broad wavelength range (~100 A) depends on
the spectral region that is examined and can be as high as 60 m/s at 5600 A and
as low as 7 m/s in the 5900 A region. RV measurements of individual spectral
lines can show even higher RV amplitudes. The acoustic cross-sections of the
atmosphere, i.e. the phase and amplitude of the pulsations, as a function of
optical depth is found for spectral lines of Ca, Cr, Fe, La, Ce, Gd, Er and Nd.
This analysis shows that pulsation phase is variable through the atmosphere and
that Nd III lines pulsate almost 180 degrees out-of-phase with those of Nd II
features and are formed significantly higher in the stellar atmosphere. This
conclusively establishes the presence of at least one radial node to the
pulsations in the upper stellar atmosphere. The histogram of pulsational phases
for all individual spectral feature shows a bi-modal Gaussian distribution with
17% of the lines having a pulsational phase approximatels 165 degrees
out-of-phase with most other spectral lines. This is also consistent with the
presence of a radial node in the stellar atmosphere. The accumulation of phase
due to a running wave component can explain the 165 degree phase difference as
well as the broader width (by a factor of two) of one of the Gaussian
components of the phase distribution.
