Ionic Interactions: Gas Phase
Ion
Chemistry and Cluster Photochemistry
James M. Farrar
B.
1948. B.A.'70, Washington University; Ph.D. '74, University of Chicago (with Yuan-tseh Lee);
National Science Foundation Fellow '70-73, Chicago; Postdoctoral Assoc. '74-'76, University of California - Berkeley (with Bruce H. Mahan); Asst. Prof. '76-'82, Assoc. Prof. '82-'86, Prof. '86-present,
Department Chair, '97-'00, Rochester; Alfred P. Sloan Fellow '81-85;
Visiting Fellow '87-'88, Joint Institute for Laboratory Astrophysics,
University of Colorado; Fellow, American Physical Society, 1987-. Tel.
(585)-275-5834
Email to : farrar@chem.rochester.edu
The Rochester Reaction Dynamics Group
The research of the Farrar group focuses on the
reaction dynamics and photochemistry of ionic species. The objectives
of these studies are to elucidate the reactivity of isolated gas phase species important
in combustion, atmospheric and interstellar chemistry, and to understand
the transition from the gas phase toward the condensed phases through
size-dependent properties of mass-selected cluster ions. The research
employs the techniques of molecular beams, mass spectrometry, laser
spectroscopy, and ab initio quantum chemistry calculations.
Crossed Beam Studies of Ion-Molecule Collisions
A long-standing interest of the group is in elucidating the
dynamics of low energy ion-molecule reactions using crossed ion and
molecular beams. The conventional approach to learning about collision
dynamics is to measure reaction product distributions in velocity space
coordinates in the center of mass frame of reference. A typical set of
data for the OD- reaction products formed in collisions of O-
+ D2 at 1.20 eV is shown here.
The loci of velocity space points corresponding to individual product
vibrational states are circles with a common origin at the system
center of mass velocity. When lab velocity distributions, which
correspond to rays emanating from the lab coordinate origin, cut
through these circles, the intensities observed provide information
about product energy and angular distributions for OD-
products in specific quantum states.
The next figure shows a complete
representation of the center of mass distribution for
this system, shown as a three-dimensional axonometric “mountain” plot,
and as a color contour map. At 1.20 eV, the OD- reactions
products are scattered forward strongly, and the most probable product
state corresponds to v’ = 3. Appropriate integrations of the full
center of mass distributions allow us to extract product vibrational
quantum state distributions. The results of that analysis for six
collision energies ranging from 0.25 to 1.20 eV are shown here.
The O- + D2 system is surprisingly
complicated in that the high vibrational excitation of the OD-
product observed at high collision energies persists to low collision
energies. Although the low energy angular distributions for individual
vibrational states show behavior much more characteristic of a reaction
proceeding through a transient intermediate, increasing collision
energy results in a vibrational inversion, the extent of which
increases with increasing collision energy. These results will provide
incisive tests of ab initio potential energy surfaces and
quantum dynamics.
Some recent studies the
group has undertaken include:
 |
Dynamics of proton transfer in the
H3O+
+ NH3 system
|
|
Dynamics of charge transfer and
deuteron transfer in the D2O+ + C2H4
and NH3 systems
|
|
Dynamics of charge transfer and
hydride transfer in the OD+ + C2H4
system
|
|
Photodissociation of mass-selected Mg+(NH3)n
clusters
|
|
Vibrational state product resolution
in OH- + D2 ®
HOD + D-
|
Imaging Experiments
The experiments we have performed to date involve point-by-point
reconstruction of the center of mass flux distributions with
one-dimensional scans in laboratory coordinates. The possibility of
detecting all velocity space elements in a single time window promises
to increase the capabilities of the crossed beam technique
significantly.
Our lab is currently developing an instrument (schematic shown at
right) that will give us the capability to image velocity space
distributions. The diagram shows that the locus of points of
reaction products with a constant center of mass speed is a sphere
whose radius increases with time. Imaging the set of nested spheres
describing the reaction products by projecting them on a plane allows
all product velocity elements to be observed in a single time window.
An example of the state-resolved nature of this detection scheme, taken
from a study of ozone photodissociation from Paul Houston's lab, is
shown here. The concentric rings
correspond
to different vibrational states of the product O2 molecules.
The application of imaging methods based on multiplex (Fellgett)
advantage will enhance product detection sensitivity by more than an
order of magnitude.
Oxidation, electron transfer, and vibrational excitation
in the photodissociation dynamics of mass –selected clusters
The group has also directed its efforts toward
photodissociation
processes in mass–selected clusters. The group employs time of flight
(TOF) mass spectrometry (see photo - click here
for schematic) and
laser photodissociation
spectroscopy to probe the absorption spectra of clusters
produced in a laser vaporization source. Absorption spectra, acquired
by measuring photofragment intensities as a function of photolysis
wavelength at a selected mass, provide information on the electronic
properties of the clusters. Our efforts have been devoted to
understanding processes of solvent evaporation and reaction in clusters
of singly-charged alkaline earth cations solvated by the polar solvent
molecules NH3, H2O, CH3OH, CH3NH2,
and their isotopomers.
A comparison of the spectra for strontium
cations
solvated by NH3, H2O, and CH3OH and
the
analogous magnesium
systems, shows a similar pattern of behavior. Large red shifts occur
with increasing numbers of
first-shell solvent molecules, but the shifts appear to saturate when
the first shell closes. A picture appears to be emerging suggesting
that such spectra provide evidence for the formation of contact and
solvent-separated ion pair species that serve as precursors to
solvated electron formation.
Our current work is focused on studying
clusters that contain more than one Rydberg center. Our recent work has
shown that clusters formed from Sr+ solvated with at least
12 NH3
molecules acquire excess hydrogen atoms. In
deuterium-labelled clusters with the stoichiometry Sr+(ND3)nDx,
we have obtained both chemical and spectroscopic evidence that the
excess D atoms can bind in two chemically distinct sites. Spectroscopic
evidence indicates that such clusters contain a metal-D core that
accounts for a single excess deuterium atom. The remaining D atoms
are surface bound as solvated ammonium radicals having the
stoichiometry ND4(ND3)n.
Friends and Family
We enjoy the opportunity to maintain our family ties. Over the
years, we have been privileged to work with many talented students,
postdocs, and collaborators at other institutions. We maintain an
e-mail list of former group members, but we
need your assistance in keeping that list current.
Please email Jim Farrar
if you see that we do not have a current address for you. The most
recent graduates
from the group include Drs. Susan Troutman Lee, James I. Lee, David C.
Sperry, and Elizabeth Richards
O'Grady. Sue is a staff scientist at ThermoFinnigan, Jim is a design
engineer at
Stanford Research Systems, David is a research chemist at
Bausch & Lomb, and Elizabeth is a Field Service Engineer at Agilent.
Our group has had a long-standing relationship with members of the
molecular beam group at the University of Perugia
in Italy. This project has its origins in the crossed beam work of
Professor
Franco Vecchiocattivi (pictured at left with his wife,
Loretta) and his co-worker Dr.
Bruno
Brunetti of Perugia. Over the
past 12 years, our groups have exchanged a
number of useful visits. Click here
to see the University
of Perugia's molecular beams homepage.
We also keep a "group photo album", updated February 8, 2007,
which contains snapshots of former group members, collaborators, and
other friends.
Please check it out!
Representative publications:
Li Liu, Yue Li, and James M. Farrar, "Singlet and Triplet State Dynamics of Charge and Hydride Transfer Reactions of OD+ (X 3S
-)with Propyne" Int. J. Mass Spectrom. 2008, Zdenek Herman Honor Issue, accepted. Click here for a preprint.
Li Liu, Yue Li, Xiaohui Cai, Elizabeth S. Richards and James M. Farrar, "Low Energy Crossed Beam Studies of OD+ and D2O+ with C2H4: Covalent and Electrostatic Complexes", Physica Scripta, 2007, 76, C48-C55.
Li Liu, Elizabeth S. Richards, and James M. Farrar, "Hydride transfer reaction dynamics of OD+ + C3H6", J. Chem. Phys, 2007, 127, 244315.
Li Liu, Courtney Martin, and James M. Farrar, "Reaction dynamics of OH+ (X 3S
-)+ C2H2 studied with crossed beams and density functional theory calculations", J. Chem. Phys., 2006, 125, 133117.
Li Liu, Yue Li, and James M. Farrar, "Dynamics study of the reaction OH- + C2H2 ®
C2H- + H2O with crossed beams and DFT calculations", J. Chem. Phys., 2006, 124, 124317.
Yue Li, Li Liu, and James M. Farrar, “A quantum state-resolved study of the four atom reaction:
OH- (X1S
+) + D2 (X1S
g+, v = 0) ®
HOD (X1A¢, v') + D- (1S)”,
J. Phys. Chem. A, 2005, 109, 6392-6396.
Li Liu, Yue Li, and James M. Farrar, “Reaction dynamics study of O-
+ C2H2 with crossed beams and density-functional theory calculations”,
J. Chem. Phys., 2005, 123, 094304.
James M.Farrar, "Ion-Molecule Reactions", in Springer Handbook of Atomic, Molecular, and Optical Physics, edited by G. W. Drake (Springer-Verlag, New York, 2005), pp. 993-1003.
Xiaohui Cai, Yue Li, Elizabeth Richards O'Grady,and James M.
Farrar,“Experimental and theoretical studies of charge transfer and
hydride transfer in the reactions of OD+ + C2H4”
Int. J. Mass Spectrom. 2005, 241, 271-282.
James I. Lee, David C. Sperry, and James M. Farrar,“Spectroscopy and
reactivity of size-selected Mg+-
ammonia clusters”,
J. Chem. Phys. 2004, 121, 8375-8384.
Yue Li and James M. Farrar,“Reaction dynamics of H2O+
(D2O+)
+ NH3 studied with crossed molecular beams and DFT
calculations”,
J. Phys. Chem. A 2004, 108, 9876-9886.
Here is a link to the first publication of Andy
Farrar (son of JMF):
Andrew M. Farrar, Artur K. Kieres, Kathryn A. Hausknecht, Harriet de
Wit, and Jerry B. Richards, "Effects of reinforcer
magnitude on an animal model of impulsive behavior",
Behavioral Processes 2003 64, 261-271.
Li Liu, Xiaohui Cai, Yue Li, Elizabeth Richards O’Grady, and James M.
Farrar,“Experimental and theoretical studies of deuterium ion transfer
and
charge transfer between D2O+ and C2H4”,
J. Chem. Phys. 2004, 121, 3495-3506.
Yue Li and James M. Farrar, “Proton transfer dynamics of the reaction H3O+
(NH3, H2O)
NH4+ “, J.
Chem. Phys. 2004, 120, 199-205.
James M. Farrar, “Crossed Beam Methods for Ion Collisions”, in Encyclopedia of Mass Spectrometry,
Volume 1, edited by Peter B. Armentrout (Elsevier, Armsterdam, 2003),
pp. 158-174.
James M. Farrar, “Size-dependent reactivity in open shell metal-ion
polar solvent clusters: spectroscopic probes of electronic-vibration
coupling, oxidation and ionization”, Int. Rev. Phys. Chem. 2003,
22, 593-640.
James M. Farrar, “Steric and Solvent Effects in Ionic Reactions”, Science
2002, 295, 2222-2223.
Click here to go to the
Chemistry
Department homepage.
Click here to go to the University
of Rochester homepage.
Please email Jim Farrar
if you have any problems with this page.
Updated August 11, 2008
jmf
