Creating a sequential list of letters with R

Did you look at

?LETTERS

and doesn't that do what you want? Else there are paste() and related functions.

Edit: Maybe the collapse= to paste is what you need:

R> replicate(5, paste(sample(LETTERS, 10, replace=TRUE), collapse=""))
[1] "OHZBIYEFMD" "UINBOFEIXN" "UORJZATYNT" "ZNPWNBFFXJ" "ZOKYMTCDKZ"
R> 

This is what you're looking for:

> paste("This_", letters, sep="")

> [1] "This_a" "This_b" "This_c" "This_d" "This_e" "This_f" "This_g" "This_h"
  [9] "This_i" "This_j" "This_k" "This_l" "This_m" "This_n" "This_o" "This_p"
  [17] "This_q" "This_r" "This_s" "This_t" "This_u" "This_v" "This_w" "This_x"
  [25] "This_y" "This_z"