# Broadening of emission peak - "red/blue spike"

• AlexVM

#### AlexVM

As I understand, spectral emission peak can broaden due to high temperature of the emitter.
For example, emission of "cold" CO2 will have a thin peak at 4.3um while the CO2 emitted from a (very hot) rocket plume will have a broad emission at ~4um - 4.7um (called "red/blue spike")

What is the physical reason for this phenomenon?
Where can I find the right formulas?

Thanks,
Alex

## Answers and Replies

As I understand, spectral emission peak can broaden due to high temperature of the emitter.
For example, emission of "cold" CO2 will have a thin peak at 4.3um while the CO2 emitted from a (very hot) rocket plume will have a broad emission at ~4um - 4.7um (called "red/blue spike")

What is the physical reason for this phenomenon?
Where can I find the right formulas?

Thanks,
Alex
This is called Doppler broadening.

As I understand, spectral emission peak can broaden due to high temperature of the emitter.
For example, emission of "cold" CO2 will have a thin peak at 4.3um while the CO2 emitted from a (very hot) rocket plume will have a broad emission at ~4um - 4.7um (called "red/blue spike")

What is the physical reason for this phenomenon?
Where can I find the right formulas?

Thanks,
Alex

The observed effect is not due to Doppler broadening of a single line. It is actually something much more complicated.

This paper discusses the origin of this effect: http://www.dpinstruments.com/papers/5093-20_theory_jellison.pdf

In the paper above, Figure 1 shows what the spectrum should look like. Figure 8 shows the observed spectrum above a portable exhaust stack.

The blue-red spikes refer to the appearance of a ro-vibrational CO2 emission spectrum that comes from a hot emitter of CO2 radiation, whose emission spectrum passes through a colder sample of CO2.

The peaks due to transitions from high J states (high rotational states) that occur in a hot sample are the ones that have little attenuations (there is relatively little CO2 in these high energy states). The peaks due to transitions from low-J states are reabsorbed by the cold CO2 between emitter and abosrber. Because of this, the central portion of the spectrum (due to the low rotational energy CO2 states) is "missing."

From the paper above:
"An additional difficulty is caused by atmospheric absorption. In field measurements on plumes, the "pure" spectrum
often will not be observable. The emitted energy passes through some path length of air before it is received by the
spectrometer, and if one is observing plume emission from CO2 (for example), the emitted energy can be partially
absorbed by the CO2 in the air. This absorption will cause a "hole" around the line center. Only the outer edges of the
branches (the so-called red and blue spikes) will be observable. Standard laboratory techniques may not be appropriate
to remote measurements of molecular transitions that are subject to significant absorption by the intervening air."