Spectroscopy of and with Laser Pointers Joel Tellinghuisen Department of Chemistry Vanderbilt University Nashville, TN 37235.

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 …