Post on 18-Dec-2015
Spectroscopy of and with Spectroscopy of and with Laser PointersLaser Pointers
Joel Tellinghuisen
Department of ChemistryVanderbilt UniversityNashville, TN 37235
Start with the punch line …
[Gerstenkorn & Luc atlas (1978)]
Ordinate scale quantitative; source?
A key-chain red laser pointer (< 5$ bulk)
0.00
0.01
0.02
0.03
196.0 196.4 196.8 197.2 197.6 198.0 198.4
Ab
so
rba
nc
e
- 15000 cm-1
Back to the beginning. What is said …Garland, Nibler, and Shoemaker (Experiments in Physical Chemistry, 7th edit.)
Expt. 39 – (Absorption and Emission Spectra of I2) — “The use of a doubled Nd:YAG laser is particularly appealing, since this is becoming increasingly available as for example a relatively inexpensive green ‘laser pointer.’ … An unseeded Nd:YAG multimode source will produce light with a width of 0.5 to 1 cm–1 so that several I2 states will be excited. This will lead to a more complex mixture of emission doublets, but this multimode source is still suitable for this experiment.”
And what you see when three different green laser pointers (GLPs) are directed through an I2 cell …
Examine the fluorescence spectra:
Hg calibration
Fluorescence excited from
= 0 = 2 = 3
And as excited by an Ar-ion laser:
The dispersed beams from a multiline Ar-ion laser traverse the cell.
Fluorescence excited by
Ar+ 514.5 nm GLP 532 nm
A visual appreciation of the effects of optical depth (beam attenuation on right) and quenching (500 Torr Ar in cell on left).
-4 -2 0 2 4
cm-1
Representative high-resolution spectra for one model of GLP, as recorded on a CCD array.
-8 -4 0 4 8 12 16
cm-1
A
BCDE
And for two other models (A,B and C-E)
The last of these seems to be operating single-mode much of the time, but …
Modes are spaced greater than anticipated, with pattern dependent upon GLP operating power and time.
Spectra recorded on a commercial UV-Vis (Shimadzu) at 0.1-nm resolution.
from http://repairfaq.ece.drexel.edu/sam/laserssl.htm#sslafgl
[Sam's Laser FAQ, Copyright © 1994-2006, Samuel M. Goldwasser, All Rights Reserved.]
So why such range and complexity of mode structure?
Lots of optical surfaces in the cavity?
651 652 653 654nm
x 100
673 674 675 676nm
By contrast, mode structure in red laser pointers (RLPs) is simple.
The two spectra in each plot below were taken for the same RLP at different times, with strong and weak batteries in one case.
But the key-chain model showed surprisingly little side-mode emission — estimated at only ~2% in the illustrated case.
-20 -10 0 10 20 30
- 15290 cm-1
x 60
657.0 658.0 659.0 660.0
nm
3.50 V
657.0 658.0 659.0 660.0
nm
3.79 V
657.0 658.0 659.0 660.0
nm
3.86 V(cold start)
657.0 658.0 659.0 660.0
nm
3.86 V(+ 1 min)
657.0 658.0 659.0 660.0
nm
3.86 V(+ 3 min)
657.0 658.0 659.0 660.0
nm
3.86 V(long t)
657.0 658.0 659.0 660.0
nm
4.08 V
657.0 658.0 659.0 660.0
nm
4.20 V
657.0 658.0 659.0 660.0
nm
4.34 V
657.0 658.0 659.0 660.0
nm
4.34 V
657.0 658.0 659.0 660.0
nm
4.34 V
657.0 658.0 659.0 660.0
nm
4.50 V
657.0 658.0 659.0 660.0
nm
4.50 V
657.0 658.0 659.0 660.0
nm
4.50 V
657.0 658.0 659.0 660.0
nm
4.65 V
657.0 658.0 659.0 660.0
nm
4.65 V
As for the GLPs, spectral output depends on power and time (evidently temperature). Thus each RLP is tunable and scanable.
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0 20 40 60 80 100
Ab
sorb
an
ce
t (s)
PI2 = 42 mTorr
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0 20 40 60 80 100
Ab
sorb
an
ce
t (s)
PI2 =103 mTorr
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0 20 40 60 80 100
Ab
sorb
an
ce
t (s)
PI2 =228 mTorr
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0 20 40 60 80 100
Ab
sorb
an
ce
t (s)
PI2 =380 mTorr
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0 20 40 60 80 100
Ab
sorb
an
ce
t (s)
PI2 =720 mTorr
Spectroscopy with laser pointers?
Record A(t) after turning GLP off, for various PI2.
-0.02
-0.01
0.00
0.01
0.02
0.03
0 20 40 60 80 100
Ab
so
rba
nc
e
t (s)
Minimal values of A between lines provide estimates of strength of continuum absorption (mainly C X)
Results: Unprecedented precision in (continuum) at 532 nm.
-0.015
-0.010
-0.005
0.000
0.005
0.010
0.015
0 1 2 3 4
Abs
orba
nce
[I2] (10-5 mol/L)
= 84.4(1.5) L mol -1 cm-1
Similar experiments with RLP: Now we can also estimate B X transition strength from integrated lines. Pick strong doublet near 15 308.4 cm–1 [P94 in 6-5 band and R85 in 4-4]
0.00
0.01
0.02
0.03
0.04
0.05
0.06
0.07
8.3 8.4 8.5 8.6 8.7 8.8 8.9 9.0 9.1
Abs
orba
nce
- 15 300 cm-1
0.000
0.005
0.010
0.015
0.000
0.001
0.002
0 1 2 3 4 5
Abs
orb
anc
e Are
a (cm-1)
[I2] (10-5 mol/l)
= 36.84(21)
|e|2 = 1.220(13) D 2
10000
12000
14000
16000
18000
20000
22000
24000
2.00 2.50 3.00 3.50 4.00 4.50 5.00
X(0g+)A'(2u)
A(1u)
B(0u+)
C1(1u)
R (Å)
E (cm-1) 0.4
0.6
0.8
1.0
1.2
1.4
450 500 550 600 650
|e|2 (D
2)
nm
0
50
100
150
200
500 550 600 650 700
(L
mo
l-1
cm-1
)
nm
C - X
A - X
Spectroscopy results summary …