Transcript TiO2-final.ppt

The 65th International Symposium
on Molecular Spectroscopy, June 2010
The Visible Spectrum of TiO2
Xiujuan Zhuang, Timothy C. Steimle & Anh Le
Dept. Chem. & BioChem., Arizona State University, Tempe, AZ,USA*
Ramya Nagarajan, Varun Gupta and John P. Maier
Dept. of Chem. Univ. of Basel, Basel, Switzerland ‡
*Funded by: DoE-BES
‡
Swiss National Science Foundation
Motivation
Bulk material
Most widely used photo-activated catalysis:
Annual production > 5.3x106 metric tons!
Molecular
TiO2
??
{
Supported TiO2
cluster
{
B. Astronomy related:
{
A. Catalysis related:
TiO
g’-band
TiO
g-band
Our Previous Study: PCCP, vol 11, pp 2649 (2009).
REMPI, LIF, optical Stark and Dispersed LIF of 18655 cm-1 .
Next Slide
Ion counts
(000)-(000)?
Matrix isolated emission
Goals of present study:
a. Assign and simulate the optical spectra
b. Improved determination of dipole moment.
c. Lifetime measurements.
Iso shift=+17 cm-1
High res. LIF; optical Stark, dispersed LIF
Lifetime measurements
Iso shift=-38 cm-1
High-resolution LIF 18411 cm-1 band


Selected for Optical Stark measurements

Optical Stark spectroscopy on the 202 303 branch 18411 cm-1 band TiO2
LIF signal
Dispersed Fluorescence of TiO2
Note difference
LIF signal
Fluorescent Lifetimes
18470 band
Single exponential
Lifetime (sec)
Time (nano-sec)
Laser Excitation Wavenumber
No obvious pattern
Large variation in Inertial Defect, D= Ia-Ib-Ic
Dipole moment of 18411 state < that for 18470 & 18655 state
Use of Inertial Defect to Assign Spectrum
1)
Vib.Quantum #
Harmonic freq.
Coriolis coefficient
Also
Elements of Wilson “G” (Geometry) and “F” (force) matrices.
Exp. Info.
Re,, & initial guess
of assignment (for i )
Dvib (predicted)
Eq. 1
initial guess of
assignment (for i )
State
18411 cm-1
18470 cm-1
Dvib(Obs,)
1.156
0.422
1.364
Dvib(Calc.)
1.124
0.424
1.435
(0,1,2)
(1,0,0)
(1,1,0)
assignment
18655 cm-1
(1,2,0)
(0,2,2)
(0,2,0)
0
1
2
3
0
1
4
0
1
2
2
3
Trends in Radiative Lifetimes
1.90
(0,2,2)
(0,2,0)
Lifetime (-sec)
1.80
2=2 (1,2,0)
2=0
1.70
2=0
1.60
2=1
2=1
1.50
2=2
1.40
2=1
2=3
1.30
1.20
17400
17600
17800
18000
18200
2=0
2=2
18400
18600
Wavenumber
18800
19000
Predicting the spectrum- Franck-Condon factors
A1B2(1,2,3)
FCF  0,0  1 2
2
03
2
Two dimensional (2D) overlap
One dimensional (1D) overlap
integral for the a1 modes.
integral for the b2 mode.
Assuming displaced & distorted harmonic oscillators
 Analytical expressions:
X1A1(0,0,0)
“2D”: Chang. JCP 128, 174111 (2008)
Normal coordinates of upper state
Normal coordinates of lower state
“1D”: Chang. JMolSpec 1232, 1021 (2005)
Coordinate of lower state
Coordinate of upper state
Predicting spectrum- Franck-Condon factors (Cont.)
“2D” integral :
Need to relate Q(X1A1) to Q(A1B2). Duschinsky transformation:
Q1&Q2 Normal
coor. of X1A1
state
~1
~1
Q( X A1 )  JQ( A B2 )  D
Q1 & Q2 Normal
coor. of A1B2
state
J =transformation matrix obtained from Normal Coordinated
Analyses for the X1A1to A1B2 states; D=displacement vector
 1.026  0.016 
Experimental data  J  

 0.017 0.974 
 0.287 

D  
 0.640 
“1D” integral :
Need to relate Q3(X1A1) to Q3 (A1B2):
Q3(X1A1) = Q3 (A1B2) +d3
d3 = displacement of non-totally symmetric normal coordinate
rigid structure d3 =0.
FCF prediction (cont.)
(0,2,2)
(0,2,0)
0
0
1
2
3
FCFdd=.4
FCF
3=0.4
FCF dd=0
FCF
3=0
Note:
Relative
intensity
4
3
2
1
0
1
2
(1,2,0)
What we have accomplished/measured.
• Assigned and simulated the optical spectrum (for > 500 nm).
• Structure for 3 vibrational levels of A1B 2.
• Lifetimes 10 vibrational levels of A1B 2.
• Vibrational frequencies for the X1A1 and A1B 2 states.
• el for X1A1 and 3 vibrational levels of A1B 2.
What needs to be done.
• Assign and simulated the optical spectrum (for <500 nm).
• Model the vibrational dependence of lifetimes and el.
• Rationalize the use of a non-zero displacement for 3
Vibronic Coupling- 3 mode is “misbehaving”
Thank You !
Thanks to Prof. R.W. Field
for lecture notes.