This web page describes our project on collisions of fully state-selected
and oriented metastable CO with well-characterized surfaces. Key to the
project is the ability to produce intense and pure molecular beams of state-selected
metastable CO (6 eV internal electronic energy) using direct laser excitation,
in combination with the ability to sensitively detect the metastable molecules
with a high spatial and temporal resolution. The metastable CO molecules
live long enough to travel the 70 cm distance from the laser excitation
region through an hexapole focussing region, which transmits exclusively
the state-selected metastable CO molecules, to an ultrahigh vacuum surface
apparatus. The internal ro-vibrational energy as well as the kinetic energy
of the colliding CO molecules can be varied over a large range and steric
effects in the CO/surface collision event can be studied. State analysis
of CO after the collision via laser ionization or LIF makes this a true
state-to-state molecule/surface scattering study. First experiments on
the scattering of metastable CO with a LiF {100} crystal surface yield
the intriguing result that a large fraction of the metastable molecules
retain their electronic energy in the scattering process, while
translationally and rotationally being partially thermalized.
Experimental
The experimental set-up for the scattering experiments consists of a
pulsed molecular beam machine that is connected to an Ultra High Vacuum
(UHV) system as schematically indicated in Figure (see also the photo).
A mixture of 20% CO in a rare gas is expanded into vacuum to produce a
pulsed supersonic beam. Metastable CO molecules are prepared by direct
laser excitation. The +parity component of the a3Pi(v,omega,J) level is
populated via excitation on the Q2(1) line of the spin-forbidden
a3Pi<-- X1Sigma+ transition using pulsed 199 nm (v=1) or 206 nm (v=0)
radiation (about 0.5 mJ in a 150 MHz bandwidth). Subsequently, the
molecular beam enters the hexapole chamber via a 0.8 mm diameter
skimmer. Only the laser-prepared metastable CO molecules are bent
around the beamstop with the electrostatic hexapole and are focused onto
the 1.2 mm diameter diaphragm; a almost pure beam of metastable CO
molecules, essentially free of carrier gas atoms and ground state CO
molecules, enters the UHV scattering chamber. The averaged kinetic
energy of the metastable CO molecules ranges from 31 to 114 meV.
A cleaved LiF(100) crystal is mounted on a manipulator in the UHV
system, 72.5 cm downstream from the excitation region. The LiF surface
is chosen for these experiments as it is known that part of the
metastable CO(a3Pi, v=0) molecules retains their electronic energy (6.0
eV) after scattering from this surface. The vibrational energy of CO
(0.22 eV) might be conserved as well in the interaction with the
surface, as high survival probabilities have been reported for
scattering of vibrationally excited NO molecules from this same surface.
After scattering, both the vibrationally elastic and inelastic channels
are probed in a (1+1)-Resonance Enhanced Multi-Photon Ionization (REMPI)
scheme. The extraction assembly used for mass-selective ion detection
is depicted in Figure.
Both the v=0 and v=1 level in the a3Pi state can be probed using
the b3Sigma+(v=0,1)<-- a3Pi (v=0,1) transitions
around 280 nm as resonant steps.
The 280 nm radiation is
produced by frequency doubling the output of a pulsed dye laser system.
Ions are produced in a volume close to the LiF surface and are extracted
to impinge on a Micro-Channel Plate (MCP) detector. The angle of
incidence is chosen to be 51 degrees, to allow free passage of the
ionization laser close to the LiF surface.
In these experiments a Pulsed Dye Amplified (PDA) laser system
(Lambda Physik LPD3000), pumped by a Nd:YAG laser (Spectra Physics
GCR-3 with injection-seeder), is used to excite the ground
state CO molecules. The seeding of the PDA-system is performed with a
single-mode cw ring dye laser system (Spectra Physics 380), pumped by
an Ar-ion laser (Spectra Physics 2017). The output of the PDA-system
(around 618 nm) is frequency tripled in the combination of a KDP
and a BBO crystal, providing 0.5 mJ/pulse of 206 nm laser radiation in
a bandwidth of 150 MHz (FWHM).
The superior spectral brightness of this system compared to
conventional pulsed dye lasers, makes this laser ideally suited for
these experiments.
State-specific Lifetime Determination of the a3Pi
State in CO
Two different techniques have been applied to measure the lifetime of the
lowest rotational levels in the metastable a3Pi(v=0,omega=1) state of CO.
First, measurement of the absolute absorption cross-section for several
absorption lines of the a(v=0)<--X(v=0) transition yields an Einstein
coefficient of A_{0,0}=97±3 s^{-1}. In combination with the experimentally
determined branching ratios for the a<--X transition, the lifetime of
each component of the a3Pi(v=0, omega=1, J=1) Lambda-doublet is determined
to be 3.67±0.20 ms. Secondly, detection of the spin-forbidden fluorescence
at two positions in the molecular beam downstream from the excitation region
as a function of velocity of the molecules directly probes the exponential
decay. With this technique the lifetime of the lower component of the same
a3Pi(v=0, omega=1, J=1) Lambda-doublet is determined to be 3.4±0.4
ms, while for the upper component a value of 3.8±0.5 ms is found.
Two-Dimensional Imaging of Metastable CO Molecules
Direct time- and spatially-resolved detection of metastable CO molecules,
prepared in selected quantum states via pulsed laser excitation, is experimentally
demonstrated in a molecular beam machine. Characterization of the molecular
beam in terms of parallel and perpendicular velocity distributions and
rotational temperatures is performed. A direct two-dimensional (2D) demonstration
of the mass-focusing effect in binary gas mixtures is given. Two-dimensional
imaging of the spatial distribution of the metastable a3Pi CO molecules
in the beam after passage through a hexapole field is used to study hexapole
focusing performance. Structured 2D-images demonstrate the dependence of
the focusing characteristics on the magnitude of the Lambda-doubling and
on the angular dependence of the focusing force in a hexapole consisting
of cylindrical rods.
Confining CO Molecules in Stable Orbits
A scheme is presented for confinement of neutral molecules in stable orbits
on the basis of the linear Stark effect in a properly designed electrostatic
trap. Based on Stark shift measurements in electric fields up to 140 kV/cm,
an electrode configuration that is, under realistic experimental conditions,
capable of confining metastable CO(a3Pi) molecules with velocities up to
22 m/s is described. Metastable CO molecules can be laser-prepared inside
the trap and their trajectories can be directly visualized via spatially
resolved detection of their fluorescence.
State-to-state Scattering of Metastable CO Molecules
from a LiF(100) Surface
Scattering of electronically excited, state-selected metastable CO(a3Pi)
molecules from a cleaved LiF(100) surface is studied experimentally. Internal
state distributions, fluorescence profiles, Time-Of-Flight (TOF) profiles
and angular distributions of the surviving metastable CO molecules are
measured. Relative and absolute survival probabilities are determined for
various impact velocities. The dependence of translation and rotational
temperature on the velocity of the incoming beam unambiguously indicates
a direct inelastic scattering process, even though the angular distributions
are broad, both in-plane and out-of-plane. The internal state distribution
after scattering shows an overpopulation of the initially prepared Omega=1-component
relative to the other spin-components.
Scattering of Vibrationally and Electronically Excited
CO Molecules from a LiF(100) Surface
Experiments are performed in which vibrationally and electronically excited
CO(a3Pi, v=1) molecules are scattered from a LiF(100) surface. As there
is originally no population in the vibrationless level of the metastable
state, this experiment gives the unique possibility to probe the vibrationally
inelastic channel in the scattering of vibrationally excited molecules.
The vibrational deactivation probability can thus be accurately determined
and is found to be below 10^{-3} for the system under study.
A reanalysis of the k3Pi state of CO
The k3Pi state of the CO molecule is investigated in the region between
91000 and 97000 cm-1 via 1+1 Resonance Enhanced Multiphoton Ionization
spectroscopy on CO molecules prepared in a single quantum level of the
aPi (v=1) state. A new vibronic band is found which is at lower energy
than the vibrational ground state reported in the literature, leading to
a reassignment of the vibrational numbering of the k3Pi state. The rotationally
resolved spectra of the k3Pi (v=0-6) <-- aPi (v=1, J=1, Omega=1) of
12CO and 13CO have been observed and analyzed, confirming the new vibrational
labeling and providing a full set of molecular constants of the k3Pi valence
state.
Double-Resonance Spectroscopy on Triplet States
of CO
Double-resonance spectroscopy is used to investigate the v'=2 level of
the b3Sigma^+ state and perturbing high vibrational levels (v'=40--42)
of the a'Sigma^+ state of CO. Single rotational levels in the a3Pi, v=0
state are prepared via laser excitation on the Cameron band around 206
nm. Cavity ring
down absorption spectroscopy is used to determine the absolute transition
strength of this spin-forbidden transition; a value of A_{0,0}=41 ±
10 s^{-1} is obtained. A laser induced fluorescence study is performed
on the metastable triplet state molecules in the region of the 2<--0
band of the Third Positive System of CO. From the analysis of the interacting
3Sigma^+ states deperturbed rotational and vibrational constants as well
as interaction parameters of these states are obtained. Franck-Condon factors
for the transition from b3Sigma^+, v'=2 and v'=0 to the a3Pi state are
obtained from dispersion measurements.
R.T. Jongma, Th. Rasing and G. Meijer. Two-dimentional imaging of
metastable CO molecules, J. Chem. Phys. 102 (1995) 1925-1933.