In my opinion, most of them may not respond you, except: 1) You met him/her before 2) He/She knows your advisor (postdoc or PhD) So mention your advisor in your email is important. Good luck!
Be polite asking them. If he/she is really influenced and inspired by your work, he/she will be kind to help you out on the letter. I have received all of positive responses on the letters except one in India.
【在 f*********s 的大作中提到】 : Be polite asking them. If he/she is really influenced and inspired by your : work, he/she will be kind to help you out on the letter. I have received all : of positive responses on the letters except one in India.