1. NON-LINEAR THERMAL LENS SIGNAL OF (𝝙u = 6) C-H vibrational overtone transition of BENZENE IN Liquid solutions of hexane
PARASHU R. NYAUPANE, CARLOS E. MANZANARES, Department of Chemistry and Biochemistry,
Baylor University, Waco, TX, USA.

2. Titan Discovered by Christiaan Huygens in 1655.
It has dense atmosphere and hydrocarbons lakes on it’s surface.
Methane and ethane are main solvents in lakes

3. Thermal Lens Spectroscopy (TLS)
Thermal lens formation

4. Thermal Lens Signal 𝑆=(α2𝑥2𝑙)( 1 1+ 𝑡 𝑐 /2𝑡 )(-( 𝟃𝑛 𝟃𝑇 ) 1 𝑘 ) 𝑃 𝝀 𝑌𝐻
𝑆=(α2𝑥2𝑙)( 1 1+ 𝑡 𝑐 /2𝑡 )(-( 𝟃𝑛 𝟃𝑇 ) 1 𝑘 ) 𝑃 𝝀 𝑌𝐻
α2 = Absorption coefficient for vibrational transition
x2 = Composition by volume
l = Length of cell
tc = Characteristics thermal time constant( 𝑡 𝑐 = 𝑤 0 2 2𝟈Cp /4k)
𝑤 0 = Beam radius of pump laser at the center of cell
𝟈 = Density
k = Thermal conductivity
Cp= Molar heat capacity of the solvent
t = Time related to modulation frequency of the pump laser
(−𝜕𝑛/𝜕𝑇)=Negative temperature gradient of the index of refraction
P = Pump power at wavelength(λ)
YH =Heat yield

5. Thermal Lens Spectrometer

6. Benzene Molecule
u=6
u=0 H

7. Thermal Lens Signal of Benzene (∆v=6) in CCl4
Thermal lens spectra of C-H vibrational overtone (𝝙v=6) of benzene in CCl4

8. Normalized Signal vs Composition of Benzene in CCl4
Non-linear normalized thermal lens signal as a function of benzene composition in CCl4

9. Normalized Signal vs Composition of Benzene in Hexane
𝑆= 𝑥 − 𝑒 −184 𝑥 2

10. Solvent Enhancement Factor
Physical properties
Symbols
C6H6
CCl4
C6H14
Index of refraction 𝜼
1.5005
1.4595
1.372
Negative temperature gradient of the index of refraction
-(𝟃𝜼/𝟃T) (K-1)
Molar density
𝟈 ( mol. cm-3)
0.0076
Thermal conductivity
K( W m-1 K-1)
Coefficient of thermal expansion(vol.)
β(K-1)
Heat capacity at constant pressure
Cp(J mol-1 K-1)
136.1
131.75
192.63
Solvent factor(enhancement)
(-(𝟃𝜼/𝟃T)/k) (m/W)
Thermal time constant
tc (s)
0.22
0.30
0.24
Calculation of the enhancement factor as a function of the composition of benzene in solvents

11. Experimental vs Predicted Signal
Comparison of predicted TLS signal (straight line) with experimental thermal lens signal (non-linear)

12. Comparison of Signal from Different Probe Lasers
488 nm
568 nm
Comparison of nonlinear (blue dots, 488 nm probe) and linear (orange dots, 568 nm probe) normalized TLS signal as a function of the composition of benzene in hexane

13. Two Color Two Photon Absorption
One color absorption: 𝑃 𝑎𝑏𝑠 =𝑃 1− 𝑒 − 𝛼 2 𝑥 2 𝑙 =P 𝛼 2 𝑥 2 𝑙
Two color absorption: 𝑃 𝑎𝑏𝑠 ′ = 𝑃 ′ 1− 𝑒 − 𝛽 2 𝑥 2 𝑙
Total thermal signal: S = 𝐺𝑃 ( 𝛼 2 𝑥 2 𝑙 +𝐶 (1− 𝑒 − 𝛽 2 𝑥 2 𝑙 ))
C = P’/P
β2 = Absorption coefficient for electronic transition
α2 = Absorption coefficient for vibrational transition
x2 = Composition by volume
l = Length of cell
G = All the terms in TLS signal except power and absorption

14. Two Color Two Photon Absorption of Benzene
V = 6
(607 nm)
(488 nm)
(270 nm)
Energy diagram for two color absorption of benzene

15. Electronic Transition of Benzene in Hexane
Ultraviolet spectrum of the secondary band of benzene in hexane

16. Summary
The thermal lens signal of the (𝝙v=6) C-H overtone transition of benzene as function of concentration in liquid hexane and CCl4 was studied at room temperature.
Non-linearity on the TLS signal of benzene for low concentration solutions (< 0.1 composition by volume) was observed. This can be explained by a two color excitation process.
Solvent enhancement is not responsible for the non-linearity and enhancement of the TLS signal.

17. Robert A. Welch Foundation
Acknowledgement
Baylor University
Robert A. Welch Foundation

18. Thank You