Laser Spectroscopy Group
Department of Physics
NUI - University College Cork
Cork, Ireland
Research Activities
• Molecular Absorption Spectroscopy
• Luminescence Excitation Spectroscopy
• Third-order Nonlinear Susceptibilities (Zscan)
• Synthesis of Metal Nanoparticles by Laser
Ablation
• Surface Enhanced (Resonance) Raman
Spectroscopy
Molecular Absorption
Spectroscopy
• Cavity Ring-Down Spectroscopy (!)
Dye laser based (vis): Static gas, supersonic jets.
• Cavity Enhanced Absorption
Spectroscopy
Diode laser based (near IR): Static gas, flow chamber.
• Incoherent Broad-Band Cavity Enhanced
Absorption Spectroscopy
Xenon Lamp based (UV / vis / near IR): Static gas.
[S.E. Fiedler, A. Hese, A.A. Ruth; Chem. Phys. Lett. 371 (2003) 284-294.]
Cavity Ring-Down Spectroscopy
Cavity-Ring Down (CRD):
Highly sensitive direct absorption method for
species in the gas-phase.
In CRD spectroscopy the rate rather than the
absolute magnitude of a change of intensity is
determined.
Advantages: • intensity independent (in principle)
• very long path-lengths
• high spectral resolution possible
• applicable over a wide spectral range
Principle of Cavity Ring-Down (CRD)
laser pulse of a dye laser
mirror (R>0.999)
rel. intensity
1,0
I0
I1
I2 ….. In
1
I0
0,8
1
I2
0,6
0,4
In
0,2
0,0
0

no absorption
I1
40
60
time / s
80
100
I = A exp(-t/crd)
crd / s
-1
fit
20
-1

 / nm
1
crd
(1  R) c
  ( ) c

Principle of Cavity Ring-Down (CRD)
laser pulse of a dye laser
mirror (R>0.999)
rel. intensity
1,0
I0
I1
I2 ….. In
2
I0
0,8
2
I2
0,6
0,4
0,2
0,0
0

absorption
I1
40
60
time / s
80
100
I = A exp(-t/crd)
crd / s
-1
fit
20
-1

 / nm
1
crd
(1  R) c
  ( ) c

Examples of investigations using
Cavity Ring-Down Spectroscopy
• Spin forbidden transitions in aromatic thiocarbonyl
S
S
compounds (in jet and static cell)
O
O
A.A. Ruth, W.G. Doherty, R.P. Brint; Chem. Phys. Lett. 352 (2002) 191-201.
A.A. Ruth, T. Fernholz, R.P. Brint, M.W.D. Mansfield; J. Mol. Spectr. 214 (2002)
80-86.
• Fast decay dynamics in jet-cooled azulene
A.A. Ruth, E.-K. Kim, A. Hese; Phys. Chem. Chem. Phys. 22 (1999) 5121-5129.
• Nonlinear dynamics of UV multiphoton photolysis
products of gaseous naphthalene
A.A. Ruth, E.W. Gash, M. Staak, S.E. Fiedler; Phys. Chem. Chem. Phys. 4 (2002)
5217-5220.
Experimental Setup
Dye
Laser
Excimer Laser
(XeCl, 308 nm)
Probe
HR Mirror
Iris
Shutter
Pump
Vacuum
Cell
Lens
Filter
PMT
Computer
GPIB
Digital
Oscilloscope
Excimer Laser
Shutter / Lens
Dye Laser
PMT
CRD Mirror
CRD Mirror
Oscillations
8
793 s
Conditions:
561 s
6
5
Pnap
PHe
EPuls
T
494 s
483 s
4
= 0.10 mbar
= 77.5 mbar
= 21.4 mJ
= 24.1 oC
-7
10  / cm
-1
7
3
2
1
0
0
20 UV Pulses
1
2
3
time / hrs.
4
5
Investigations using Cavity Enhanced
Absorption Spectroscopy
Investigations using Cavity Enhanced
Absorption Spectroscopy
High spectral resolution (~60 MHz)
rel. absorbance
• Water vapour overtones in ambient air (vis)
• Formaldehyde (~1.5 m) at 4 mbar
1511.4 nm
20
1510.8 nm
rotationally resolved
15
10
5
6616.5
6617.0
6617.5
-1
~
 / cm
6618.0
6618.5
Z-scans of Pt-octaethylporphyrin
in toluene
Open aperture:
|  | = (3.97±0.09) x 10-9 m W-1
Closed aperture:
’= 7.7 x 10-16 m2 W-1
1.3
norm. transmission
norm. transmission
1.4
1.3
1.2
1.1
1.2
1.1
1.0
0.9
1.0
0
10
20
30
40
50
z / mm
Pt-OEP in toluene (8.6 x 10-5 M)
0.8
0.0
0.5
1.0
1.5
2.0
2.5
z / mm
Energy per pulse 8 J.
3.0
Synthesis of metal
nanoparticles by laser ablation
irises
·····
········
·
Nd:YAG
····
·
· ·····
·
532 nm
metal (target)
x
cell
H2 O
y
z
optical density
lens
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
200
400
600
 / nm
800
<d>~30 nm
Group Members
 Academic Staff
Prof. M. Mansfield (general supervision)
 Post-Doctoral Fellows
To be appointed
( CEAS and IBBCEAS – multi-component trace gas detection)
 PhD Students
E. Gash (CRD, nonlinear dynamics of naphthalene)
M. Staak (CEA, formaldehyde, photolysis products - HO2)
K. Lynch (Surface enhanced Raman scattering)
One position vacant
 Masters Students
S. O’Brien (z-scan measurements of porphyrin solutions)
A. Walsh (z-scan measurements of porphyrin thin films)
[ R. Healy (Nanoparticle formation by laser ablation – alloys) ]
Research Collaborations
 University College Cork – Physics Department
Prof. D. Nikogosyan, Dr. A. Dragomir
(nonlinear absorption measurements)
 University College Cork – Chemistry Department
Prof. P. Brint (LIF, LIP, forbidden transitions, supersonic jets)
Prof. J. Sodeau, Dr. J. Wenger (atmospheric chemistry)
Dr. J. Holmes, Dr. M. Morris, Prof. T. Spalding
(nanoparticles and nanostructures, TEM)
Prof. D. Burke (laser assisted electrocatalysis)
 Technical University Berlin, Germany
Prof. A. Hese, S. Fiedler (CRD spectroscopy, IBBCEAS)
 Max-Planck-Inst. Biophys. Chemistry, Göttingen, Germany
Prof. J. Troe (liquid phase spectroscopy and kinetics)
Univ. Paris-Sud (Dr. Orphal), Niels-Bohr-Inst. Copenhagen (Prof.
Heimburg), Warsaw University (Dr. Borowicz) …. etc.
Future activities
 Multi-component gas analysis using Incoherent BroadBand Cavity Enhanced Absorption Spectroscopy
Detection of NO2, NO3, O3 , H2O, HONO … (vis)
 Alloy nanoparticle synthesis
New materials for SERS
 Surface Enhanced (Resonance) Raman Scattering
Porphyrins and proteins on particle surfaces