  A coherent vibrational wavepacket is launched and manipulated in the
symmetric stretch (a$_1$) mode of CBr$_4$, by impulsive stimulated Raman
scattering from non-resonant 400 nm laser pump pulses with various peak
intensities on the order of tens of 10$^{12}$ W/cm$^2$. Extreme ultraviolet
(XUV) attosecond transient absorption spectroscopy (ATAS) records the
wavepacket dynamics as temporal oscillations in XUV absorption energy at the
bromine M$_{4,5}$ 3d$_{3/2,5/2}$ edges around 70 eV. The results are augmented
by nuclear time-dependent Schr\"odinger equation simulations. Slopes of the
(Br-3d$_{3/2,5/2}$)$^{-1}$10a$_1^*$ core-excited state potential energy surface
(PES) along the a$_1$ mode are calculated to be -9.4 eV/{\AA} from restricted
open-shell Kohn-Sham calculations. Using analytical relations derived for the
small-displacement limit with the calculated slopes of the core-excited state
PES, a deeper insight into the vibrational dynamics is obtained by retrieving
the experimental excursion amplitude of the vibrational wavepacket and the
amount of population transferred to the vibrational first-excited state, as a
function of pump-pulse peak intensity. Experimentally, the results show that
XUV ATAS is capable of easily resolving oscillations in the XUV absorption
energy on the order of few to tens of meV and tens of femtosecond time
precision, limited only by the averaging times in the experimental scans. This
corresponds to oscillations of C-Br bond length on the order of 10$^{-4}$ to
10$^{-3}$ {\AA}. The results and the analytic relationships offer a clear
physical picture, on multiple levels of understanding, for how the pump-pulse
intensity controls the vibrational dynamics launched by non-resonant ISRS in
the small-displacement limit.
