On the Stark Effect of the O I 777-nm Triplet in Plasma and Laser Fields

Round 1

Reviewer 1 Report

“On the Stark effect of the O I 777-nm triplet in plasma and laser fields”

In this paper Evgeny Stambulchik et al., have calculated the plasma Stark broadening of the O I 777-nm line using modern techniques, and to investigate the influence of the laser field on the shape of this line using a computer simulation (CS) modeling.

The results of the plasma Stark broadening closely match those of Griem. Have confirmed that a moderate laser radiation causes a noticeable shift of all triplet components (at 777.19, 777.42, and 777.54 nm) and, with an appropriate distribution of the magnitudes, may result in effective broadening and shift that could be misattributed to a higher plasma density.

As the Stark triplet amplification constant is used for the diagnosis of plasmas in many investigations, I consider that the article and obtained results are interesting in order to be publishable.x

Author Response

We thank the Referee for reviewing our manuscript.

Reviewer 2 Report

This work is devoted to a detailed study of the phenomena of Stark broadening for the triplet transition OI 1777 nm under the action of a plasma microfield and a laser field with different frequencies close to the structure of atomic energy levels. The study revealed several interesting features in the dependence of the line width and shift on the temperature of the plasma electrons. Noteworthy is the analysis of the additive contribution of the highly excited 4s, 3d energy states to the total line shift. An analysis of the contribution of the laser magnetic field to the observed line shifts is also of fundamental interest. On the whole, the article met the quality conditions for publication in Atom.

There are several comments on the content of the article:

• The authors used computer simulation (CS) in their calculations; it is not entirely clear what this means for the problems of line broadening, namely, molecular dynamics simulation of particle motion with or without correlation effects? This should be clearly explained in the text.
• It seems that the dependence of the population on time in Fig. 4 should approach statistical weight at such a high electron density; This is true? Please provide some comments on this.

Comments do not change the positive assessment of the article.

Author Response

We thank the Referee for reviewing our manuscript and providing valuable comments.

Below, we answer specific points.

The authors used computer simulation (CS) in their calculations; it is not entirely
clear what this means for the problems of line broadening, namely, molecular
dynamics simulation of particle motion with or without correlation effects? This
should be clearly explained in the text.

Indeed, a description of the MD simulation was erroneously omitted; we now added a new paragraph (lines 44-51 in the revised manuscript) with proper references etc.

It seems that the dependence of the population on time in Fig. 4 should approach
statistical weight at such a high electron density; This is true? Please provide some
comments on this.

Yes, you are right. It is, in fact, the same point as one raised by another Referee and caused by our omission to clearly explain that the results presented in Fig.4 correspond to a single run of the code. After having averaged over a statistically meaningful number of runs, the population histories indeed approach the statistical weights. As an example, please see the figure attached showing this for population of the 3s level.

The following clarifying text was already included in the previous revision:

We note that the population histories given here are obtained in a single run of the code. After having averaged over many runs, one would observe familiar smooth relaxation curves establishing statistical equilibrium.

Sincerely yours,

Evgeny Stambulchik /on behalf of all authors/

Author Response File: Author Response.pdf

Reviewer 3 Report

In the paper the authors study the O I 777-nm triplet transition which is very important because of its use for plasma density diagnostics. The transition was reported that have the spectral line anomalously broadened in some laser-produced plasma. It disagrees with previously published data for its spectral line width in common plasmas. Their study was just aimed to resolve this controversy. They employed a computer simulation in order to analyze the effect of broadening in plasma and in plasma in the presence of laser electromagnetic microfield. They showed that the presence of electromagnetic laser microfield has significant impact on this triplet transition and shift and width of this observed spectral line. In addition they emphasised the importance of using time-dependent density matrix in their calculation.

I have only minor questions for authors:

I noticed discrete steps in function of populations of electronic states in Figure 4. (lower panel). I suppose that this has only numerical sense because that you use an approximate approach for density matrix in your calculations. If you used greater number of runs in calculations and shorter time period, or if you calculated for longer time period before the laser pulse, would the steps in the population be more smoothed?

I propose that the authors add one or two sentences in the text to clarifying this.

Author Response

We thank the Referee for reviewing our manuscript and providing valuable comments.

Below, we answer specific points.

I noticed discrete steps in function of populations of electronic states in Figure 4. (lower panel). I suppose that this has only numerical sense because that you use an approximate approach for density matrix in your calculations.

No, the solution is numerically exact in this case. As briefly mentioned in the text, these jumps are due to the so called "strong collisions" with electrons having a very small impact parameter; even though the number of such electrons is small (say, ~1%), they contribute strongly to the plasma kinetics and line-shape formation. In between these strong impacts, the evolution of the atomic system is governed by the weak overlapping collisions with majority of the electrons and slow, mostly adiabatic, fields of ions (and the laser field, of course).

If you used greater number of runs in calculations and shorter time period, or if you calculated for longer time period before the laser pulse, would the steps in the population be more smoothed?

We indeed did not clearly state that the results presented in Fig.4 correspond to a single run of the code; this is likely the source of the confusion. After having averaged over a statistically meaningful number of runs, the population histories become smoothed. As an example, please see the figure attached showing this for population of the 3s level.

I propose that the authors add one or two sentences in the text to clarifying this.

Thank you, we have done it. Specifically, the added text reads:

We note that the population histories given here are obtained in a single run of the code. After having averaged over many runs, one would observe familiar smooth relaxation curves establishing statistical equilibrium.

Sincerely yours,

Evgeny Stambulchik /on behalf of all authors/

Author Response File: Author Response.pdf