Open Question: Need help in writing a job application thank you letter?

an HR has been email contacting me and pre-screened me, but when i went to on-site interview the interviewer is someone higher than her and i did not get a chance to meet her. the interviewer also forgot to bring business card and he said i can contact the HR. so now i want to write a thank you letter to the HR and ask for the interviewer's email address to write him a thank you letter as well. so for someone who i haven't really talked to and interviewed with how can i write a more meaningful thank you note? plz help me as i am a ESL student!!~~~