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

Start with the punch line … [Gerstenkorn & Luc atlas (1978)]  Ordinate scale quantitative; source?  A key-chain red laser pointer (< 5$ bulk)

Back to the beginning. What is said … Garland, Nibler, and Shoemaker (Experiments in Physical Chemistry, 7th edit.) Expt. 39 – (Absorption and Emission Spectra of I 2 ) — “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 I 2 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 I 2 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 nmGLP 532 nm

A visual appreciation of the effects of optical depth (beam attenuation on right) and quenching (500 Torr Ar in cell on left).

Representative high-resolution spectra for one model of GLP, as recorded on a CCD array.

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 exel.edu/sam/laserssl. htm#sslafgl exel.edu/sam/laserssl. htm#sslafgl [Sam's Laser FAQ, Copyright © , Samuel M. Goldwasser, All Rights Reserved.] So why such range and complexity of mode structure? Lots of optical surfaces in the cavity?

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.

As for the GLPs, spectral output depends on power and time (evidently temperature). Thus each RLP is tunable and scanable.

Spectroscopy with laser pointers? Record A(t) after turning GLP off, for various P I 2.

Minimal values of A between lines provide estimates of strength of continuum absorption (mainly C  X)

Results: Unprecedented precision in  (continuum) at 532 nm.

Similar experiments with RLP: Now we can also estimate B  X transition strength from integrated lines. Pick strong doublet near cm –1 [P94 in 6-5 band and R85 in 4-4]

Spectroscopy results summary …