Python熊猫-如何转换字符串HH:MM:SS。嗯sss.ms浮动?

I would like to convert string value HH:MM:SS.mmm to float value sec.milliseconds for arithmetic calculations. Is there a direct way to do this as currently I am doing it via split string function and it is a very tedious process. Dataframe looks like:I would like to convert string value HH:MM:SS.m