SEM Results: The Herzberg Gratings

Dr. Gerhard Herzberg was a German physicist who emigrated to the University of Saskatchewan in 1935. He later went to the Yerkes Observatory but, following the Second World War, was lured back to Canada’s National Research Council by the promise of a new, fully-equipped lab. One of the first pieces of equipment built in the lab was a three-metre (ten-foot) vacuum spectrograph with which Herzberg could mimic conditions in Space. With this spectrograph (1993.0323) he was able, in 1958, to identify CH₂: the first free radical found in Space. This led to a Nobel Prize in Chemistry in 1971. Key to Herzberg’s spectroscopic studies were the gratings. Three (of nine) were imaged at the CCI.

Herzberg’s three-metre (ten-foot) vacuum spectrograph (CSTM 1993.0323) in his lab at the National Research Council of Canada in Ottawa

1993.0321 is the oldest of the gratings from Herzberg’s lab which have been tested. Knowledge of Herzberg’s wide web of contacts and its characteristics suggests that this grating may have been made by Harold Babcock. There are also similarities to the Babcock ruling 1996.0247: i.e., the number of grooves/mm are similar, both are blazed, and both are fairly regular, although there is more irregularity on the sides of the blazed grooves of 1993.0321. These irregularities, however, are less than 1µm. Just left of centre, and running through all of the groove tops, we can see lighter patches, suggesting a flaw on the surface of the glass block into which the grooves were ruled. The ruled area is approx. 8 x 4 centimetres (3 x 1.5 inches); there are smaller ruled “test” areas above and below: a unique feature of this grating.

At right, grating (CSTM 1993.0321) and mount made for Herzberg, possibly by Harold Babcock

Gratings 1993.0315 and 1993.0316 were both made by Bausch & Lomb in Rochester, New York. 1993.0315 was purchased in 1960 for $357.50. 1993.0315 is blazed (36° 52'), but the image is difficult to interpret. Clearly, the diamond was not “dressed” or polished very well, leaving the fine lines parallel to, and within, the grooves. It appears that the dark lines are the bottoms of the grooves, and the fainter lines immediately beneath them are the tops of the grooves. The glass is smooth with few flaws.

At right, Bausch & Lomb grating (CSTM 1993.0315) and mount from Herzberg’s lab

By contrast, grating 1993.0316 has a haze over its entire surface. This is caused by light reflecting off the many tiny irregular scallops on the edges of the grooves, and shows that the fused quartz substrate was not as smoothly ruled as one would hope. This may be related to the low blazing angle of 4° 45', and would have resulted in poorly focused spectral lines. The spots visible at high magnification on this grating may be related to the magnesium-fluoride (MgF₂) anti-reflective overcoating.