Laser Spectroscopy of Radicals, Carbenes and Ions in
Superfluid Helium Droplets
70th International Symposium on Molecular Spectroscopy
Coblentz Award Lecture
6/24/15
Gary E. Douberly
Department of Chemistry, University of Georgia
Athens, Georgia, USA
Program Scope and Goals
• Mid-IR spectroscopy of molecular radicals relevant to combustion
and atmospheric chemistry
• Provide a starting point for high-resolution gas-phase studies
• Challenge emerging electronic structure methods
• Rational in situ synthesis of reaction intermediates
• Formation and spectroscopy of entrance and exit channel openshell molecular complexes
“pick-up cells”
~1011 molecules·cm-3
Mass Spectrometry
Laser spectroscopy
Stark/Zeeman spectroscopy
Trapping Potential Depth ~𝟏𝟎𝟎 K
Molecular degrees of freedom are brought into thermal equilibrium
with the helium droplet at 𝑻 = 𝟎. 𝟒 K
“pick-up cells”
~1011 molecules·cm-3
T=0.4 K
Cooling timescale < 10 ns, pick-up timescale ~10 s
10000 He atoms can dissipate ~6 eV (~140 kcal/mol)
He droplets are optically transparent below ~20 eV.
Spectroscopic study of the outcome of cold collisions between sequentially
picked-up reactants (both reactive and non-reactive collisions)
Kinetic Trapping of Metastable Clusters
Rapid cooling of the condensing molecular system
Long range forces can steer “reactants” into local minima
G.E. Douberly and R.E. Miller J. Chem. Phys, 2005, 122, 024306
Molecular Radical and Carbene Production
“pick-up cells”
molecules·cm-3
Pyrolysis of Organic Precursors
He Droplet Source 11
~10
F.P. Lossing, Canadian J. Chem., 49, 357 (1971)
Pick-up cells
H2O cooled Cu electrodes
Quartz Pyrolysis Tube
cw-“Chen” Nozzle
Molecular Radical and Carbene Production
“pick-up cells”
He Droplet Source 11
~10 molecules·cm-3
Pick-up cells
Methyl
(CH3)
J. Phys. Chem. A (2013), 117, 11640.
Vinyl
(C2H3)
J. Chem. Phys. (2013), 138, 174302.
Ethyl
(C2H5)
J. Chem. Phys. (2013), 138, 194303.
Propargyl (C3H3) and (C3H3OO)
J. Phys. Chem. A (2013), 117, 13626.
Allyl
(C3H5) and (C3H3OO) J. Chem. Phys. (2013), 139, 234301.
Hydroxyl (OH)
J. Phys. Chem. A (2013), 117, 8103.
Hydridotrioxygen (HOOO) J.C.P. (2012), 137, 184302; J.P.C. Lett (2013), 4, 3584.
OH(C2H2)
J.C.P. (2015), 142, 134306; J.M.S. (2015) accepted.
OH(CH3OH)
J. Phys. Chem. A, (2015) accepted.
Hydroxymethylene (HCOH)
J. Chem. Phys. (2014), 140, 171102.
Dihydroxymethylene (HOCOH) J. Chem. Phys. (2015), 142, 144309.
(He)3+
Helium Droplet Detection via Mass
Spectrometry
(He)n+
e- (70 eV)
+ +
+
++
(He)
He2 n (n ≥ 2)
2e-
Helium Droplet Detection via Mass
Spectrometry
(He)3+
(He)n+
(H2O)+
e- (70 eV)
+ +
+
+
H 2O
IP difference (He – molecule) ≈ 12-15 eV
2e-
Helium Droplet Detection via Mass
Spectrometry (1-butyne-4-nitrite)
He+ +
{
}+*
39 (C3H3+)
(C3H3)+ + NO + CH2O
IP difference (He – molecule) ≈ 12-15 eV
Propargyl Radical via 1-butyne-4-nitrite Pyrolysis
32
Add O2 to downstream PUC
30 (H2CO+, NO+)
38 (C3H2+)
+
39 (C3H3+)
Neat droplet beam
+
Infrared cw-OPO
h
1000 K
m/z=38 depletion
38
300 K
39
m/z=39 depletion
m/z=38 detection
 = -1.4 cm-1
BGas = 2730 MHz
BHe = 1020 MHz
Trot = 0.37 K
The Microscopic Andronikashvili Experiment
OCS in a
pure 4He droplet
Increasing
number of 4He
atoms
OCS in a
pure 3He droplet
Narrow Linewidths ~200 MHz
Inhomogeneous Broadening
Mechanisms Dominate
Grebenev, Toennies, Vilesov, Science 279, 2084 (1998)
The Microscopic Andronikashvili Experiment
Conclusion:
The appearance of rotational
fine structure is a
microscopic manifestation
of 4He superfluidity.
Grebenev, Toennies, Vilesov, Science 279, 2084 (1998)
OCS
Group theory and the gas-phase
effective Hamiltonian approach
still work!
Bgas / BHe=2.2
Exactly what we expect for an (a1) band of a C3v symmetric top
2B
2
2B
1
𝐻 = 𝐻𝑟𝑜𝑡 + 𝐻𝑆𝑂 + 𝐻𝐶𝐷 + 𝐻𝑞
|𝐽, 𝑃, 𝜆, 𝜎, 𝜖
1
=
|𝐽, 𝑃, 𝜆, 𝜎 + 𝜖 −1
2
(𝐽−1/2) |𝐽, −𝑃, −𝜆, −𝜎
OH
C2H2
Detect laserinduced depletion
of ionization crosssection
cw-OPO (idler 3m)
Droplet Beam
EStark
Elaser
M = 0
EStark
or
Elaser
M = ±1
Accepted last week
Infrared Stark Spectroscopy
EStark
Elaser
M = ±1
OH
C2H2
Detect laserinduced depletion
of ionization crosssection
1 Tesla Rare Earth
Permanent Magnets
(iron caps)
EZeeman
Joe Brice still has all
10 fingers!
Elaser
M = 0
EZeeman
or
Elaser
M = ±1
Infrared Zeeman Spectroscopy
EZeeman
Elaser
EZeeman
Elaser
M = ±1
M = 0
Infrared Zeeman Spectroscopy
Perpendicular polarization
∆𝑀 = ±1
OHCO
2
3/2
0.425 Tesla
Parallel polarization
∆𝑀 = 0
Wavenumbers (cm-1)
Wavenumbers (cm-1)
VPT2
f
h
S. Davis, D. Uy, and D. J. Nesbitt, J. Chem. Phys. 112, 1823 (2000)
P. M. Johnson and T. J. Sears, J. Chem. Phys. 111, 9222 (1999)
100 ps
5 ps
resonance
polyad
Infrared Spectra of the n- and i-propyl radicals
Christopher Moradi
Schriener et.al. Angew. Chem. Int. Edit. (2008), 47, 7071.
b
b
a
a
(a1)
(a)
(b2)
Stark Spectroscopy of trans,cis-dihydroxycarbene
3649
3650
3651
3652
3651
3652
20.050 kV/cm
b
a
5.017 kV/cm
𝝁𝒂 = 𝟏. 𝟔𝟑(𝟑) 𝐃
𝝁𝒃 = 𝟏. 𝟓𝟎(𝟓) 𝐃
0 kV/cm
EStark
Elaser
M = 0
3649
3650
-1
Wavenumber (cm )
Stark Spectroscopy of trans,trans-dihydroxycarbene
3658
3659
3660
3661
3660
3661
36.377 kV/cm
b
a
30.003 kV/cm
𝝁𝒂 = 𝟎 𝐃
𝝁𝒃 = 𝟎. 𝟔𝟖(𝟔) 𝐃
0 kV/cm
EStark
Elaser
M = 0
3658
3659
-1
Wavenumber (cm )
Bimolecular Reactions in
Helium Droplets?
Spectroscopic Detection of
C3H3OO
65%
35%
CCSD(T)/ANO spin density calculations: 65% C(1), 35% C(3)
E.B. Jochnowitz…J.F. Stanton, G.B. Ellison JPC A 109, 3812, (2005).
Resonance stabilization Energy ~11 kcal/mol
High concentrations in flames; self reaction first step to soot formation
~2 kcal/mol
~5 kcal/mol
QCISD(T) calculations
Barrier heights need to be reduced
to reproduce experimental rate constants
Sequential pick-up of C3H3 and O2
~2 kcal/mol
~5 kcal/mol
QCISD(T) calculations
Sequential pick-up of C3H3 and O2
+ O2 →
∆H19 kcal/mol
~1500 He atoms
m/z=27 u
Depends strongly on O2
C3H3OO
C3H3
Mass Spec (Laser OFF – ON)
(C2H3+ + CO2)
32
39
P.S. Thomas, N.D. Kline, T.A. Miller, J. Phys. Chem. A 114, 12437 (2010)
acetylenic-trans isomer (2A′′)
Solid Argon: 3326 cm-1
CCSD(T) VPT2: 3332 cm-1
Barrier Heights?
“…given the high spin-contamination at these
entrance saddlepoints, it would be worthwhile, in
future calculations, to perform a multi-reference based
analysis…” D. K. Hahn, S. J. Klippenstein, and J. A. Miller
J. Phys. Chem. A (2013), 117, 13626.
Summary
* Pyrolytic decomposition of organic precursors combined with
mass spectrometry allows for the spectroscopic study of a broad
range of molecular radicals and carbenes within helium droplets.
* Low temperature and isotropic environment allows for a detailed
study of the vibrational complexity associated with hydrocarbon
radicals relevant IONS
to combustion.
in Helium Droplets!!
Gert von Helden
often exhibit rotational fine
* Small radical and carbene systems
WG01
structure in the IR spectra, and these can be probed with Stark
and Zeeman spectroscopy.
* Bimolecular reactions can be carried out within the helium
droplet, and the products or intermediates associated with these
reactions are carried downstream.
(so far… Methyl + O2 / Ethyl + O2 / Allyl + O2 / Propargyl + O2)
Acknowledgments
Post-Docs: Paul Raston (JMU), Christopher Leavitt (UGA), Bernadette Broderick (UGA)
Graduate Students: Alexander Morrison, Tao Liang, Christopher Moradi, Joseph Brice
Collaborators: Mark Marshall, John Stanton, Henry F. Schaefer, Wesley Allen, Jay Agarwal,
Stephen Klippenstein
Support:
University of Georgia
AFRL Eglin Air Force Base
ACS-Petroleum Research Fund
U.S. National Science Foundation (CAREER)
U.S. Department of Energy, Office of Science (BES-GPCP)