IR + UV laser pulse control of momenta directed to specific products:
Quantum simulations for HOD*--->H+OD versus HO+D
Publication:
Nadia Elghobashi, Pascal Krause, Jörn Manz and Markus Oppel
IR + UV laser pulse control of momenta directed to specific products:
Quantum simulations for HOD*--->H+OD versus HO+D
Phys.Chem.Chem.Phys., 5, 4806-4813
(2003)
Introduction, concepts, and motivation:
Femtosecond ( 1 fs = 0.000 000 000 000 001 seconds!) infrared (IR) and
ultraviolet (UV) laser pulses may be applied in order to achieve selective
bond breaking in molecules. A new scenario for this type of IR + UV laser
control of chemical reactions has been suggested in the above publication and is
illustrated in the following movies. The animations show quantum simulations
of the laser driven molecular wavepackets which represent the selective bond
breaking.
Explanation and Results:
The following movies show the potential energy surfaces of the electronic
ground (bottom) and excited state (top) of deuterated water HOD, viewed
along the OH (left) and OD (right) bonds, together with the laser driven wavepackets
moving on these potential energy surfaces.
Klick to download movie I.
As a reference simulation, an ultrashort UV laser pulse (without IR pulse)
excites the HOD from the well of the potential energy surface representing HOD in its electronic ground state to the domain close to the barrier of the
potential energy surface of HOD in the excited state. The corresponding
molecular wavepacket is transferred in a "vertical" so called
Franck-Condon type transition. As soon as it arrives on the electronic
excited state, it separates due to the potential barrier into two
partial waves which evolve along opposite dissociation channels of the potential energy
surface, representing the competing products H+OD (left channel) and HO+D
(right channel). No selectivity is achieved by means of pure UV laser pulse
excitation. The real time of the mechanism is only approx. 10 fs!
Klick to download movie II.
An ultrashort IR laser is applied, polarized essentially along the OH bond
of the pre-oriented model system, HOD in the electronic ground state. The
IR laser frequency is chosen near resonant to the frequency of the local
HO stretch vibration of HOD. As a consequence, the OH stretch starts to
vibrate with increasing amplitudes as well as increasing values of momenta
directed along the OH stretch. The value of the momentum is a maximum when
the wavepacket passes through the minimum of the potential well. In this
moment, the ultrashort UV laser pulse is applied (the same pulse type as shown in the first movie). The vertical Franck-Condon type transition transfers
the wavepacket with momentum directed along the OH bond from the
electronic ground state to the excited state. As a consequence, the
molecular wavepacket is excited to the same position as in the first
movie, but with specific momentum which drives the wavepacket towards
increasing OH bond stretches, yielding the selective products H+OD, not
HO+D.
Klick to download movie III.
Blow-up of the second movie, emphasizing the event of the vertical
Franck-Condon type transition with momentum directed along the OH stretch.
Acknowledgments:
We are grateful to Profs. I.V. Hertel and N.E. Henriksen
for stimulating discussions, to Profs. V. Engel, R. Schinke and V.
Staemmler for providing us with their potential energy surface, and to the
Deutsche Forschungsgemneinschaft (project Sfb 450) and Fonds der
Chemischen Industrie for financial support. The computer simulations have
been carried out by Pascal Krause on our HP workstations and on the SGI 3400 computerserver
at the computing center (ZEDAT) of the Freie Universität Berlin.
|