APS March Meeting 2012
Volume 57, Number 1
Monday–Friday, February 27–March 2 2012;
Boston, Massachusetts
Session A34: Focus Session: Impact of Ultrafast Lasers I: X-rays and THz
8:00 AM–10:48 AM,
Monday, February 27, 2012
Room: 107A
Sponsoring
Unit:
DCP
Chair: Nancy Levinger and Amber Kummel, Colorado State University
Abstract ID: BAPS.2012.MAR.A34.5
Abstract: A34.00005 : Ultrafast molecular processes mapped by femtosecond x-ray diffraction
9:36 AM–10:12 AM
Preview Abstract
Abstract
Author:
Thomas Elsaesser
(Max-Born-Institute, 2 a Max-Born-St., Berlin, 12489)
X-ray diffraction with a femtosecond time resolution allows for
mapping photoinduced structural dynamics on the length scale of a
chemical bond and in the time domain of atomic and molecular motion.
In a pump-probe approach, a femtosecond excitation pulse induces
structural changes which are probed by diffracting a femtosecond
hard x-ray pulse from the excited sample. The transient angular
positions and intensities of diffraction peaks give insight into the
momentary atomic or molecular positions and into the distribution of
electronic charge density. The simultaneous measurement of changes
on different diffraction peaks is essential for determining atom
positions and charge density maps with high accuracy. Recent
progress in the generation of ultrashort hard x-ray pulses (Cu
K$_{\alpha}$, wavelength $\lambda=0.154$ nm) in laser-driven plasma
sources has led to the implementation of the powder diffraction and
the rotating crystal method with a time resolution of 100 fs. In
this contribution, we report new results from powder diffraction
studies of molecular materials. A first series of experiments gives
evidence of a so far unknown concerted transfer of electrons and
protons in ammonium sulfate [(NH$_4$)$_2$SO$_4$], a centrosymmetric
structure. Charge transfer from the sulfate groups results in the
sub-100 fs generation of a confined electron channel along the
c-axis of the unit cell which is stabilized by transferring protons
from the adjacent ammonium groups into the channel. Time-dependent
charge density maps display a periodic modulation of the channel's
charge density by low-frequency lattice motions with a concerted
electron and proton motion between the channel and the initial
proton binding site. A second study addresses atomic rearrangements
and charge dislocations in the non-centrosymmetric potassium
dihydrogen phosphate [KH$_2$PO$_4$, KDP]. Photoexcitation generates
coherent low-frequency motions along the LO and TO phonon
coordinates, leaving the average atomic positions unchanged. The
time-dependent maps of electron density demonstrate a concomitant
oscillatory relocation of electronic charge with a spatial amplitude
of the order of a chemical bond length, two orders of magnitude
larger than the vibrational amplitudes. The coherent phonon motions
drive the charge relocation, similar to a soft mode driven phase
transition between the ferro- and paraelectric phase of KDP.
To cite this abstract, use the following reference: http://meetings.aps.org/link/BAPS.2012.MAR.A34.5