plpgsql; execute query inside exists

Hi there,

I would like to use EXISTS in a small plpgsql function but I always
get a "syntax error". How can I execute a query inside the
EXISTS function?

IF NOT EXISTS(EXECUTE 'SELECT * FROM '|| tmp_tbl)
THEN
CREATE TABLE tt();

ERROR: syntax error at or near "EXECUTE"
LINE 1: SELECT NOT EXISTS(EXECUTE 'SELECT * FROM '|| $1 )

Sorry for my lame question but I'm new in postgres.

Thanks!